- A description of an event or system that is considered to be accurate.
- A coherent statement or set of ideas that explains observed facts or phenomena and correctly predicts new facts or phenomena not previously observed, or which sets out the laws and principles of something known or observed; a hypothesis confirmed by observation, experiment etc.
- The underlying principles or methods of a given technical skill, art etc., as opposed to its practice.
- A field of study attempting to exhaustively describe a particular class of constructs.
- A hypothesis or conjecture.
- A set of axioms together with all statements derivable from them; or, a set of statements which are deductively closed. Equivalently, a formal language plus a set of axioms (from which can then be derived theorems). The statements may be required to all be bound (i.e., to have no free variables).

- acoustic theory
- algorithmic information theory
- antenna theory
- atomic theory
- catastrophe theory
- category theory
- cell theory
- chaos theory
- circuit theory
- complexity theory
- computation theory
- control theory
- critical theory
- decision theory
- domino theory
- extreme value theory
- game theory
- giant impact theory
- graph theory
- group theory
- information theory
- kinetic theory of gases
- knot theory
- literary theory
- M-theory
- music theory
- number theory
- opponent-process theory
- phlogiston theory
- probability theory
- proof theory
- quantum field theory
- rational choice theory
- set theory
- signal theory
- social theory
- spectral theory
- systems theory
- theory of gravity
- theory of relativity
- theory of truth
- Theory X
- Theory Y
- type theory
- value theory
- virtue theory

- From Middle French théorie, from Late Latin theōria, from Ancient Greek θεωρία, from θεωρέω, from θεωρός, from θέα + ὁράω [i. e. θέαν ὁράω (théan horáō, “see, look at a view; survey + genitive”)].

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