Definition

Definition

Overview
A definition is a passage that explains the meaning of a term (a word
Word
In language, a word is the smallest free form that may be uttered in isolation with semantic or pragmatic content . This contrasts with a morpheme, which is the smallest unit of meaning but will not necessarily stand on its own...

, phrase
Phrase
In everyday speech, a phrase may refer to any group of words. In linguistics, a phrase is a group of words which form a constituent and so function as a single unit in the syntax of a sentence. A phrase is lower on the grammatical hierarchy than a clause....

 or other set of symbol
Symbol
A symbol is something which represents an idea, a physical entity or a process but is distinct from it. The purpose of a symbol is to communicate meaning. For example, a red octagon may be a symbol for "STOP". On a map, a picture of a tent might represent a campsite. Numerals are symbols for...

s), or a type of thing. The term to be defined is the definiendum. A term may have many different senses or meanings. For each such specific sense, a definiens is a cluster of words that defines that term.

A chief difficulty in managing definition is the need to use other terms that are already understood or whose definitions are easily obtainable.
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Encyclopedia
A definition is a passage that explains the meaning of a term (a word
Word
In language, a word is the smallest free form that may be uttered in isolation with semantic or pragmatic content . This contrasts with a morpheme, which is the smallest unit of meaning but will not necessarily stand on its own...

, phrase
Phrase
In everyday speech, a phrase may refer to any group of words. In linguistics, a phrase is a group of words which form a constituent and so function as a single unit in the syntax of a sentence. A phrase is lower on the grammatical hierarchy than a clause....

 or other set of symbol
Symbol
A symbol is something which represents an idea, a physical entity or a process but is distinct from it. The purpose of a symbol is to communicate meaning. For example, a red octagon may be a symbol for "STOP". On a map, a picture of a tent might represent a campsite. Numerals are symbols for...

s), or a type of thing. The term to be defined is the definiendum. A term may have many different senses or meanings. For each such specific sense, a definiens is a cluster of words that defines that term.

A chief difficulty in managing definition is the need to use other terms that are already understood or whose definitions are easily obtainable. The use of the term in a simple example may suffice. By contrast, a dictionary definition has additional details, typically including an etymology
Etymology
Etymology is the study of the history of words, their origins, and how their form and meaning have changed over time.For languages with a long written history, etymologists make use of texts in these languages and texts about the languages to gather knowledge about how words were used during...

 showing snapshots of the earlier meanings and the parent language.

Like other words, the term definition has subtly different meanings in different contexts. A definition may be descriptive of the general use meaning, or stipulative of the speaker's immediate intentional meaning. For example, in formal languages like mathematics, a 'stipulative' definition guides a specific discussion. A descriptive definition can be shown to be "right" or "wrong" by comparison to general usage, but a stipulative definition can only be disproved by showing a logical contradiction.

A precising definition
Precising definition
A precising definition is a definition that extends the lexical definition of a term for a specific purpose by including additional criteria that narrow down the set of things meeting the definition....

 extends the descriptive dictionary definition (lexical definition) of a term for a specific purpose by including additional criteria that narrow down the set of things meeting the definition.

C.L. Stevenson
Charles Stevenson
Charles Leslie Stevenson was an American analytic philosopher best known for his work in ethics and aesthetics....

 has identified persuasive definition
Persuasive definition
A persuasive definition is a form of definition which purports to describe the 'true' or 'commonly accepted' meaning of a term, while in reality stipulating an uncommon or altered use, usually to support an argument for some view, or to create or alter rights, duties or crimes.The terms thus...

 as a form of stipulative definition which purports to describe the "true" or "commonly accepted" meaning of a term, while in reality stipulating an altered use, perhaps as an argument for some specific view.

Stevenson has also noted that some definitions are "legal" or "coercive", whose object is to create or alter rights, duties or crimes.

Intension and extension


An intensional definition
Intensional definition
In logic and mathematics, an intensional definition gives the meaning of a term by specifying all the properties required to come to that definition, that is, the necessary and sufficient conditions for belonging to the set being defined....

, also called a coactive definition, specifies the necessary and sufficient conditions
Necessary and sufficient conditions
In logic, the words necessity and sufficiency refer to the implicational relationships between statements. The assertion that one statement is a necessary and sufficient condition of another means that the former statement is true if and only if the latter is true.-Definitions:A necessary condition...

 for a thing being a member of a specific set. Any definition that attempts to set out the essence of something, such as that by genus and differentia
Genus-differentia definition
A genus–differentia definition is a type of intensional definition, and it is composed of two parts:# a genus : An existing definition that serves as a portion of the new definition; all definitions with the same genus are considered members of that genus.# the differentia: The portion of the new...

, is an intensional definition.

An extensional definition
Extensional definition
An extensional definition of a concept or term formulates its meaning by specifying its extension, that is, every object that falls under the definition of the concept or term in question....

, also called a denotative definition, of a concept or term specifies its extension
Extension (semantics)
In any of several studies that treat the use of signs - for example, in linguistics, logic, mathematics, semantics, and semiotics - the extension of a concept, idea, or sign consists of the things to which it applies, in contrast with its comprehension or intension, which consists very roughly of...

. It is a list naming every object
Object (philosophy)
An object in philosophy is a technical term often used in contrast to the term subject. Consciousness is a state of cognition that includes the subject, which can never be doubted as only it can be the one who doubts, and some object or objects that may or may not have real existence without...

 that is a member of a specific set.

So, for example, an intensional definition of 'Prime Minister
Prime minister
A prime minister is the most senior minister of cabinet in the executive branch of government in a parliamentary system. In many systems, the prime minister selects and may dismiss other members of the cabinet, and allocates posts to members within the government. In most systems, the prime...

' might be the most senior minister of a cabinet in the executive branch of government in a parliamentary system. An extensional definition would be a list of all past, present and future prime minister
Prime minister
A prime minister is the most senior minister of cabinet in the executive branch of government in a parliamentary system. In many systems, the prime minister selects and may dismiss other members of the cabinet, and allocates posts to members within the government. In most systems, the prime...

s.

One important form of the extensional definition is ostensive definition
Ostensive definition
An ostensive definition conveys the meaning of a term by pointing out examples. This type of definition is often used where the term is difficult to define verbally, either because the words will not be understood or because of the nature of the term...

. This gives the meaning of a term by pointing, in the case of an individual, to the thing itself, or in the case of a class, to examples of the right kind. So you can explain who Alice (an individual) is by pointing her out to me; or what a rabbit (a class) is by pointing at several and expecting me to 'catch on'. The process of ostensive definition itself was critically appraised by Ludwig Wittgenstein
Ludwig Wittgenstein
Ludwig Josef Johann Wittgenstein was an Austrian philosopher who worked primarily in logic, the philosophy of mathematics, the philosophy of mind, and the philosophy of language. He was professor in philosophy at the University of Cambridge from 1939 until 1947...

.

An enumerative definition
Enumerative definition
An enumerative definition of a concept or term is a special type of extensional definition that gives an explicit and exhaustive listing of all the objects that fall under the concept or term in question...

 of a concept or term is an extensional definition
Extensional definition
An extensional definition of a concept or term formulates its meaning by specifying its extension, that is, every object that falls under the definition of the concept or term in question....

 that gives an explicit and exhaustive listing of all the object
Object (philosophy)
An object in philosophy is a technical term often used in contrast to the term subject. Consciousness is a state of cognition that includes the subject, which can never be doubted as only it can be the one who doubts, and some object or objects that may or may not have real existence without...

s that fall under the concept or term in question. Enumerative definitions are only possible for finite sets and only practical for relatively small sets.

Divisio and partitio


Divisio and partitio are classical
Classics
Classics is the branch of the Humanities comprising the languages, literature, philosophy, history, art, archaeology and other culture of the ancient Mediterranean world ; especially Ancient Greece and Ancient Rome during Classical Antiquity Classics (sometimes encompassing Classical Studies or...

 terms for definitions. A partitio is simply an intensional definition. A divisio is not an extensional definition. Divisio is an exhaustive list of subset
Subset
In mathematics, especially in set theory, a set A is a subset of a set B if A is "contained" inside B. A and B may coincide. The relationship of one set being a subset of another is called inclusion or sometimes containment...

s of a set, in the sense that every member of the "divided" set is a member of one of the subsets. An extreme form of divisio lists all sets whose only member is a member of the "divided" set. The difference between this and an extensional definition is that extensional definitions list members, and not sets.

Definition by genus and differentia


A genus–differentia definition is a type of intensional definition
Intensional definition
In logic and mathematics, an intensional definition gives the meaning of a term by specifying all the properties required to come to that definition, that is, the necessary and sufficient conditions for belonging to the set being defined....

, and it is composed of two parts:
  1. a genus
    Genus
    In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

     (or family): An existing definition that serves as a portion of the new definition; all definitions with the same genus are considered members of that genus.
  2. the differentia
    Differentia
    In Scholastic logic, differentia is one of the predicables. It is that part of a definition which is predicable in a given genus only of the definiendum....

    : The portion of the new definition that is not provided by the genera.

For example, consider these two definitions:
  • a triangle
    Triangle
    A triangle is one of the basic shapes of geometry: a polygon with three corners or vertices and three sides or edges which are line segments. A triangle with vertices A, B, and C is denoted ....

    : A plane figure that has 3 straight bounding sides.
  • a quadrilateral
    Quadrilateral
    In Euclidean plane geometry, a quadrilateral is a polygon with four sides and four vertices or corners. Sometimes, the term quadrangle is used, by analogy with triangle, and sometimes tetragon for consistency with pentagon , hexagon and so on...

    : A plane figure that has 4 straight bounding sides.

Those definitions can be expressed as one genus and two differentiae:
  1. one genus: A plane figure.
  2. two differentiae:
    • the differentia for a triangle: that has 3 straight bounding sides.
    • the differentia for a quadrilateral: that has 4 straight bounding sides.


When multiple definitions could serve equally well, then all such definitions apply simultaneously. For instance, given the following:
  • a rectangle
    Rectangle
    In Euclidean plane geometry, a rectangle is any quadrilateral with four right angles. The term "oblong" is occasionally used to refer to a non-square rectangle...

    : a quadrilateral that has interior angles which are all right angles.
  • a rhombus
    Rhombus
    In Euclidean geometry, a rhombus or rhomb is a convex quadrilateral whose four sides all have the same length. The rhombus is often called a diamond, after the diamonds suit in playing cards, or a lozenge, though the latter sometimes refers specifically to a rhombus with a 45° angle.Every...

    : a quadrilateral that has bounding sides which all have the same length.

both of these definitions of 'square' are equally acceptable:
  • a square: a rectangle that is a rhombus.
  • a square: a rhombus that is a rectangle.

Thus, a 'square' is a member of both the genus 'rectangle' and the genus 'rhombus'. In such a case, it is notationally convenient to consolidate the definitions into one definition that is expressed with multiple genera (and possibly no differentia, as in the following):
  • a square: a rectangle and a rhombus.

or completely equivalently:
  • a square: a rhombus and a rectangle.

Rules for definition by genus and differentia


Certain rules have traditionally been given for this particular type of definition.
  1. A definition must set out the essential attributes of the thing defined.
  2. Definitions should avoid circularity. To define a horse as 'a member of the species equus would convey no information whatsoever. For this reason, Locking adds that a definition of a term must not comprise of terms which are synonymous with it. This would be a circular definition, a circulus in definiendo. Note, however, that it is acceptable to define two relative terms in respect of each other. Clearly, we cannot define 'antecedent' without using the term 'consequent', nor conversely.
  3. The definition must not be too wide or too narrow. It must be applicable to everything to which the defined term applies (i.e. not miss anything out), and to nothing else (i.e. not include any things to which the defined term would not truly apply).
  4. The definition must not be obscure. The purpose of a definition is to explain the meaning of a term which may be obscure or difficult, by the use of terms that are commonly understood and whose meaning is clear. The violation of this rule is known by the Latin term obscurum per obscurius. However, sometimes scientific and philosophical terms are difficult to define without obscurity. (See the definition of Free will
    Free will
    "To make my own decisions whether I am successful or not due to uncontrollable forces" -Troy MorrisonA pragmatic definition of free willFree will is the ability of agents to make choices free from certain kinds of constraints. The existence of free will and its exact nature and definition have long...

     in Wikipedia, for instance).
  5. A definition should not be negative where it can be positive. We should not define 'wisdom' as the absence of folly, or a healthy thing as whatever is not sick. Sometimes this is unavoidable, however. We cannot define a point except as 'something with no parts', nor blindness except as 'the absence of sight in a creature that is normally sighted'.

Essence


In classical thought, a definition was taken to be a statement of the essence of a thing. Aristotle
Aristotle
Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...

 had it that an object's essential attributes form its "essential nature", and that a definition of the object must include these essential attributes.

The idea that a definition should state the essence of a thing led to the distinction between nominal and real essence, originating with Aristotle. In a passage from the Posterior Analytics
Posterior Analytics
The Posterior Analytics is a text from Aristotle's Organon that deals with demonstration, definition, and scientific knowledge. The demonstration is distinguished as a syllogism productive of scientific knowledge, while the definition marked as the statement of a thing's nature, .....

, he says that we can know the meaning of a made-up name (he gives the example 'goat stag'), without knowing what he calls the 'essential nature' of the thing that the name would denote, if there were such a thing. This led medieval logicians to distinguish between what they called the quid nominis or 'whatness of the name', and the underlying nature common to all the things it names, which they called the quid rei or 'whatness of the thing'. (Early modern philosophers like Locke used the corresponding English terms 'nominal essence' and 'real essence'). The name 'hobbit
Hobbit
Hobbits are a fictional diminutive race who inhabit the lands of Middle-earth in J. R. R. Tolkien's fiction.Hobbits first appeared in the novel The Hobbit, in which the main protagonist, Bilbo Baggins, is the titular hobbit...

', for example, is perfectly meaningful. It has a quid nominis. But we could not know the real nature of hobbits, even if there were such things, and so we cannot know the real nature or quid rei of hobbits. By contrast, the name 'man' denotes real things (men) that have a certain quid rei. The meaning of a name is distinct from the nature that thing must have in order that the name apply to it.

This leads to a corresponding distinction between nominal and real definition. A nominal definition is the definition explaining what a word means, i.e. which says what the 'nominal essence' is, and is definition in the classical sense as given above. A real definition, by contrast, is one expressing the real nature or quid rei of the thing.

This preoccupation with essence dissipated in much of modern philosophy. Analytic philosophy
Analytic philosophy
Analytic philosophy is a generic term for a style of philosophy that came to dominate English-speaking countries in the 20th century...

 in particular is critical of attempts to elucidate the essence of a thing. Russell
Bertrand Russell
Bertrand Arthur William Russell, 3rd Earl Russell, OM, FRS was a British philosopher, logician, mathematician, historian, and social critic. At various points in his life he considered himself a liberal, a socialist, and a pacifist, but he also admitted that he had never been any of these things...

 described it as "a hopelessly muddle-headed notion".

More recently Kripke's
Kripke semantics
Kripke semantics is a formal semantics for non-classical logic systems created in the late 1950s and early 1960s by Saul Kripke. It was first made for modal logics, and later adapted to intuitionistic logic and other non-classical systems...

 formalisation of possible world
Possible world
In philosophy and logic, the concept of a possible world is used to express modal claims. The concept of possible worlds is common in contemporary philosophical discourse and has also been disputed.- Possibility, necessity, and contingency :...

 semantics in modal logic
Modal logic
Modal logic is a type of formal logic that extends classical propositional and predicate logic to include operators expressing modality. Modals — words that express modalities — qualify a statement. For example, the statement "John is happy" might be qualified by saying that John is...

 led to a new approach to essentialism
Essentialism
In philosophy, essentialism is the view that, for any specific kind of entity, there is a set of characteristics or properties all of which any entity of that kind must possess. Therefore all things can be precisely defined or described...

. Insofar as the essential properties of a thing are necessary to it, they are those things it possesses in all possible worlds. Kripke refers to names used in this way as rigid designator
Rigid designator
In modal logic and the philosophy of language, a term is said to be a rigid designator when it designates the same thing in all possible worlds in which that thing exists and does not designate anything else in those possible worlds in which that thing does not exist...

s.

Recursive definitions


A recursive definition
Recursive definition
In mathematical logic and computer science, a recursive definition is used to define an object in terms of itself ....

, sometimes also called an inductive definition, is one that defines a word in terms of itself, so to speak, albeit in a useful way. Normally this consists of three steps:
  1. At least one thing is stated to be a member of the set being defined; this is sometimes called a "base set".
  2. All things bearing a certain relation to other members of the set are also to count as members of the set. It is this step that makes the definition recursive
    Recursion
    Recursion is the process of repeating items in a self-similar way. For instance, when the surfaces of two mirrors are exactly parallel with each other the nested images that occur are a form of infinite recursion. The term has a variety of meanings specific to a variety of disciplines ranging from...

    .
  3. All other things are excluded from the set


For instance, we could define natural number
Natural number
In mathematics, the natural numbers are the ordinary whole numbers used for counting and ordering . These purposes are related to the linguistic notions of cardinal and ordinal numbers, respectively...

 as follows (after Peano
Peano axioms
In mathematical logic, the Peano axioms, also known as the Dedekind–Peano axioms or the Peano postulates, are a set of axioms for the natural numbers presented by the 19th century Italian mathematician Giuseppe Peano...

):
  1. "0" is a natural number.
  2. Each natural number has a distinct successor, such that:
    • the successor of a natural number is also a natural number, and
    • no natural number is succeeded by "0".
  3. Nothing else is a natural number.


So "0" will have exactly one successor, which for convenience we can call "1". In turn, "1" will have exactly one successor, which we would call "2", and so on. Notice that the second condition in the definition itself refers to natural numbers, and hence involves self-reference
Self-reference
Self-reference occurs in natural or formal languages when a sentence or formula refers to itself. The reference may be expressed either directly—through some intermediate sentence or formula—or by means of some encoding...

. Although this sort of definition involves a form of circularity
Circular definition
A circular definition is one that uses the term being defined as a part of the definition or assumes a prior understanding of the term being defined. Either the audience must already know the meaning of the key term, or the definition is deficient in including the term to be defined in the...

, it is not vicious
Vicious circle principle
The vicious circle principle is a principle that was endorsed by many predicativist mathematicians in the early 20th century to prevent contradictions. The principle states that no object or property may be introduced by a definition that depends on that object or property itself...

, and the definition has been quite successful.

Working definitions


A working definition is either chosen for an occasion and may not fully conform with established or authoritative definitions. Not knowing of established definitions would be grounds for selecting or devising a working definition. Or it refers to a definition being developed; a tentative definition that can be tailored to create an authoritative definition.

Limitations of definition


Given that a natural language
Natural language
In the philosophy of language, a natural language is any language which arises in an unpremeditated fashion as the result of the innate facility for language possessed by the human intellect. A natural language is typically used for communication, and may be spoken, signed, or written...

 such as English
English language
English is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...

 contains, at any given time, a finite number of words, any comprehensive list of definitions must either be circular or rely upon primitive notion
Primitive notion
In mathematics, logic, and formal systems, a primitive notion is an undefined concept. In particular, a primitive notion is not defined in terms of previously defined concepts, but is only motivated informally, usually by an appeal to intuition and everyday experience. In an axiomatic theory or...

s. If every term of every definiens must itself be defined, "where at last should we stop?" A dictionary, for instance, insofar as it is a comprehensive list of lexical definition
Lexical definition
The lexical definition of a term, also known as the dictionary definition, is the meaning of the term in common usage. As its other name implies, this is the sort of definition one is likely to find in the dictionary...

s, must resort to circularity
Symbol grounding
The Symbol Grounding Problem is related to the problem of how words get their meanings, and hence to the problem of what meaning itself really is. The problem of meaning is in turn related to the problem of consciousness, or how it is that mental states are meaningful...

.

Many philosophers have chosen instead to leave some terms undefined. The scholastic philosophers
Scholasticism
Scholasticism is a method of critical thought which dominated teaching by the academics of medieval universities in Europe from about 1100–1500, and a program of employing that method in articulating and defending orthodoxy in an increasingly pluralistic context...

 claimed that the highest genera (the so-called ten generalissima) cannot be defined, since we cannot assign any higher genus under which they may fall. Thus we cannot define being
Being
Being , is an English word used for conceptualizing subjective and objective aspects of reality, including those fundamental to the self —related to and somewhat interchangeable with terms like "existence" and "living".In its objective usage —as in "a being," or "[a] human being" —it...

, unity and similar concepts. Locke
John Locke
John Locke FRS , widely known as the Father of Liberalism, was an English philosopher and physician regarded as one of the most influential of Enlightenment thinkers. Considered one of the first of the British empiricists, following the tradition of Francis Bacon, he is equally important to social...

 supposes in An Essay Concerning Human Understanding
An Essay Concerning Human Understanding
First appearing in 1690 with the printed title An Essay Concerning Humane Understanding, An Essay Concerning Human Understanding by John Locke concerns the foundation of human knowledge and understanding. He describes the mind at birth as a blank slate filled later through experience...

 that the names of simple concepts do not admit of any definition. More recently Bertrand Russell
Bertrand Russell
Bertrand Arthur William Russell, 3rd Earl Russell, OM, FRS was a British philosopher, logician, mathematician, historian, and social critic. At various points in his life he considered himself a liberal, a socialist, and a pacifist, but he also admitted that he had never been any of these things...

 sought to develop a formal language based on logical atoms
Logical atomism
Logical atomism is a philosophical belief that originated in the early 20th century with the development of analytic philosophy. Its principal exponents were the British philosopher Bertrand Russell, the early work of his Austrian-born pupil and colleague Ludwig Wittgenstein, and his German...

. Other philosophers, notably Wittgenstein
Ludwig Wittgenstein
Ludwig Josef Johann Wittgenstein was an Austrian philosopher who worked primarily in logic, the philosophy of mathematics, the philosophy of mind, and the philosophy of language. He was professor in philosophy at the University of Cambridge from 1939 until 1947...

, rejected the need for any undefined simples. Wittgenstein pointed out in his Philosophical Investigations
Philosophical Investigations
Philosophical Investigations is, along with the Tractatus Logico-Philosophicus, one of the most influential works by the 20th-century philosopher Ludwig Wittgenstein...

 that what counts as a "simple" in one circumstance might not do so in another. He rejected the very idea that every explanation of the meaning of a term needed itself to be explained: "As though an explanation hung in the air unless supported by another one", claiming instead that explanation of a term is only needed when we need to avoid misunderstanding.

Locke and Mill
John Stuart Mill
John Stuart Mill was a British philosopher, economist and civil servant. An influential contributor to social theory, political theory, and political economy, his conception of liberty justified the freedom of the individual in opposition to unlimited state control. He was a proponent of...

 also argued that we cannot define individuals
Principle of individuation
The Principle of Individuation is a criterion which supposedly individuates or numerically distinguishes the members of the kind for which it is given, i.e. by which we can supposedly determine, regarding any kind of thing, when we have more than one of them or not. It is also known as a...

. We learn names by connecting an idea with a sound, so that speaker and hearer have the same idea when the same word is used. This is not possible when no one else is acquainted with the particular thing that has "fallen under our notice". Russell offered his theory of descriptions
Theory of descriptions
The theory of descriptions is the philosopher Bertrand Russell's most significant contribution to the philosophy of language. It is also known as Russell's Theory of Descriptions...

 in part as a way of defining a proper name, the definition being given by a definite description
Definite description
A definite description is a denoting phrase in the form of "the X" where X is a noun-phrase or a singular common noun. The definite description is proper if X applies to a unique individual or object. For example: "the first person in space" and "the 42nd President of the United States of...

 that "picks out" exactly one individual. Saul Kripke
Saul Kripke
Saul Aaron Kripke is an American philosopher and logician. He is a professor emeritus at Princeton and teaches as a Distinguished Professor of Philosophy at the CUNY Graduate Center...

 pointed to difficulties with this approach, especially in relation to modality
Modal logic
Modal logic is a type of formal logic that extends classical propositional and predicate logic to include operators expressing modality. Modals — words that express modalities — qualify a statement. For example, the statement "John is happy" might be qualified by saying that John is...

, in his book Naming and Necessity.

There is a presumption in the classic example of a definition that the definiens can be stated. Wittgenstein argued that for some terms this is not the case. The examples he used include game, number and family. In such cases, he argued, there is no fixed boundary that can be used to provide a definition. Rather, the items are grouped together because of a family resemblance
Family resemblance
Family resemblance is a philosophical idea made popular by Ludwig Wittgenstein, with the best known exposition being given in the posthumously published book Philosophical Investigations It has been suggested that Wittgenstein picked the idea and the term from Nietzsche, who had been using it,...

. For terms such as these it is not possible and indeed not necessary to state a definition; rather, one simply comes to understand the use of the term.

In medicine


In medical dictionaries, definitions should to the greatest extent possible be:
  • simple and easy to understand, preferably even by the general public;
  • useful clinically or in related areas where the definition will be used;
  • specific, that is, by reading the definition only, it should ideally not be possible to refer to any other entity than the definiendum;
  • measurable;
  • reflecting current scientific knowledge.

See also

  • Analytic proposition
  • Definable set
    Definable set
    In mathematical logic, a definable set is an n-ary relation on the domain of a structure whose elements are precisely those elements satisfying some formula in the language of that structure...

  • Definitionism
    Definitionism
    Definitionism is the school of thought in which it is believed that a proper explanation of a theory consists of all the concepts used by that theory being well-defined. This approach has been criticized for its dismissal of the importance of ostensive definitions....

  • Ostensive definition
    Ostensive definition
    An ostensive definition conveys the meaning of a term by pointing out examples. This type of definition is often used where the term is difficult to define verbally, either because the words will not be understood or because of the nature of the term...

  • Demonstration
    Demonstration
    Demonstration may refer to:* Demonstration , a political rally or protest* Demonstration , a conclusive mathematical proof* Demonstration , a method of teaching by example rather than simple explanation...

  • Extensional definition
    Extensional definition
    An extensional definition of a concept or term formulates its meaning by specifying its extension, that is, every object that falls under the definition of the concept or term in question....

  • Fallacies of definition
    Fallacies of definition
    Fallacies of definition refer to the various ways in which definitions can fail to have merit. The term is used to suggest analogy with the logical fallacies...

  • Circular definition
    Circular definition
    A circular definition is one that uses the term being defined as a part of the definition or assumes a prior understanding of the term being defined. Either the audience must already know the meaning of the key term, or the definition is deficient in including the term to be defined in the...

  • Indeterminacy
    Indeterminacy (Philosophy)
    Indeterminacy, in philosophy, can refer both to common scientific and mathematical concepts of uncertainty and their implications and to another kind of indeterminacy deriving from the nature of definition or meaning...

  • Intensional definition
    Intensional definition
    In logic and mathematics, an intensional definition gives the meaning of a term by specifying all the properties required to come to that definition, that is, the necessary and sufficient conditions for belonging to the set being defined....

  • Lexical definition
    Lexical definition
    The lexical definition of a term, also known as the dictionary definition, is the meaning of the term in common usage. As its other name implies, this is the sort of definition one is likely to find in the dictionary...

  • Ramsey–Lewis method
  • Semantic
  • Synthetic proposition
  • Theoretical definition
    Theoretical definition
    A theoretical definition gives the meaning of a word in terms of the theories of a specific discipline. This type of definition assumes both knowledge and acceptance of the theories that it depends on. To theoretically define is to create a hypothetical construct...


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