theory

noun

1. (countable, uncountable)A description of an event or system that is considered to be accurate.

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

2. (countable, uncountable)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.

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

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

   “It was only when Einstein's theory of relativity was published in 1915 that physicists could show that Mercury's "anomaly" was actually because Newton's gravitational theory was incomplete.”

   “The world would need additional decades [...] before the Big Bang would begin to move from interesting idea to established theory.”

   “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.

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

4. (countable, uncountable)A field of study attempting to exhaustively describe a particular class of constructs.

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

   “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?”

   “The theory is that by stripping costs to the bone, they are able to offer ludicrously low fares.”

5. (countable)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).

   “A theory is consistent if it has a model.”

6. (countable, uncountable)The standardization and study of fixed sequences of moves, especially in the opening phase of a game.

   “Near-synonym: joseki”

7. (countable, obsolete, uncountable)Mental conception; reflection, consideration.
8. (countable, informal, uncountable)A hypothesis or conjecture.