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”)].

• (Received Pronunciation, General Australian) IPA: /ˈθɪə.ɹi/, /ˈθiː.ə.ɹi/, /ˈθɪɹ.i/
• (General American) IPA: /ˈθɪɚ.i/, /ˈθi.ə.ɹi/

theory

1. A description of an event or system that is considered to be accurate.
   • 1646, Thomas Browne, Pseudodoxia Epidemica, VII.19:

     As they encrease the hatred of vice in some, so doe they enlarge the theory of wickednesse in all.

2. (sciences) 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. [from 17th c.]
   • 1843, John Stuart Mill, ""A System of Logic, Ratiocinative and Inductive, ..., Bk V, Ch 7:

     In its most proper acceptation, theory means the completed result of philosophical induction from experience.

   • 1990, Tony Bennett, Outside Literature, page 139:

     Does this mean, then, that there can be no such thing as a theory of literature?

   • 2009, Richard Dawkins, The Greatest Show On Earth: The Evidence for Evolution, Bantam, page 10:

     Scientists and creationists are understanding the word "theory" in two very different senses. Evolution is a theory in the same sense as the heliocentric theory. In neither case should the word "only" be used, as in "only a theory".

3. (uncountable) The underlying principles or methods of a given technical skill, art etc., as opposed to its practice. [from 17th c.]
   • 1998, Elizabeth Souritz, The Great History of Russian Ballet:

     Lopukhov wrote a number of books and articles on ballet theory, as well as his memoirs.

4. (mathematics) A field of study attempting to exhaustively describe a particular class of constructs. [from 18th c.]

   Knot theory classifies the mappings of a circle into 3-space.

5. A hypothesis or conjecture. [from 18th c.]
   • 1999, Wes DeMott, Vapors:

     It's just a theory I have, and I wonder if women would agree. But don't men say a lot about themselves when a short-skirted woman slides out of a car or chair?

6. (countable, logic) A set of axioms together with all statements derivable from them; or, a set of statements which are Deductive closure. 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).

   A theory is consistent if it has a model.

7. (obsolete) Mental conception; reflection, consideration. [16th]

• See also Thesaurus:supposition

• French: théorie
• German: Theorie
• Italian: teoria
• Portuguese: teoria
• Russian: тео́рия
• Spanish: teoría

• French: théorie
• German: Theorie
• Italian: teoria
• Portuguese: teoria
• Russian: тео́рия
• Spanish: teoría

• French: théorie
• German: Theorie
• Italian: teoria
• Portuguese: teoria
• Russian: тео́рия
• Spanish: teoría

• German: Theorie
• Italian: teoria
• Portuguese: teoria
• Russian: тео́рия
• Spanish: teoría