The second declension is a category of nouns in Latin

Latin

Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 and Greek

Ancient Greek

Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...

 with similar case

Grammatical case

In grammar, the case of a noun or pronoun is an inflectional form that indicates its grammatical function in a phrase, clause, or sentence. For example, a pronoun may play the role of subject , of direct object , or of possessor...

 formation

Inflection

In grammar, inflection or inflexion is the modification of a word to express different grammatical categories such as tense, grammatical mood, grammatical voice, aspect, person, number, gender and case...

. In particular, these nouns are thematic, with an original o in most of their forms. In Classical Latin

Classical Latin

Classical Latin in simplest terms is the socio-linguistic register of the Latin language regarded by the enfranchised and empowered populations of the late Roman republic and the Roman empire as good Latin. Most writers during this time made use of it...

 the short o of the nominative and accusative singular became u.

Both Latin and Greek have two basic classes of second-declension nouns: masculine and neuter. Most masculines have -us (Latin) or -ος -os (Greek) in the nominative singular, except for the r-stem nouns in Latin, and the "Attic" declension and contracted declension in Attic Greek

Attic Greek

Attic Greek is the prestige dialect of Ancient Greek that was spoken in Attica, which includes Athens. Of the ancient dialects, it is the most similar to later Greek, and is the standard form of the language studied in courses of "Ancient Greek". It is sometimes included in Ionic.- Origin and range...

. The neuter nominative/accusative singular ends with -um (Latin) and -ον -on (Greek), matching the masculine accusative.

See Also

For specifics on the second declension as it appears in Latin and Greek, see the appropriate sections in Latin declension and Ancient Greek nouns.

The Wiktionary appendix Second declension contains more detailed information and full paradigm tables for the Latin second declension.