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(1)   A tentative theory about the natural world; a concept that is not yet verified but that if true would explain certain facts or phenomena
"A scientific hypothesis that survives experimental testing becomes a scientific theory"
"He proposed a fresh theory of alkalis that later was accepted in chemical practices"
(2)   A belief that can guide behavior
"The architect has a theory that more is less"
"They killed him on the theory that dead men tell no tales"
(3)   A well-substantiated explanation of some aspect of the natural world; an organized system of accepted knowledge that applies in a variety of circumstances to explain a specific set of phenomena
"Theories can incorporate facts and laws and tested hypotheses"
"True in fact and theory"


English since 1613, from < < < < < + .


  1. An unproven conjecture.
    I have a theory about who broke into the school last night, but I have no proof to back it up.
  2. An expectation of what should happen, barring unforeseen circumstances.
    So we’ll be there in three hours?That’s the theory.
  3. A coherent statement or set of statements that attempts to explain observed phenomena.
    There is now a well-developed theory of electrical charge.
  4. A logical structure that enables one to deduce the possible results of every experiment that falls within its purview.
    The theory of relativity was proposed by Einstein.
  5. A field of study attempting to exhaustively describe a particular class of constructs.
    Knot theory classifies the mappings of a circle into 3-space.
  6. A set of axioms together with all statements derivable from them.
    A theory is consistent if it has a model.