Etimology
From Middle French théorie, from Late Latin theōria, from Ancient Greek θεωρία (theōría, “contemplation, speculation, a looking at, things looked at”), from θεωρέω (theōréō, “I look at, view, consider, examine”), from θεωρός (theōrós, “spectator”), from θέα (théa, “view”) + ὁράω (horáō, “I see, look”) [i. e. θέαν ὁράω (théan horáō, “see, look at a view; survey + genitive”)].
noun
theory (countable and uncountable, plural theories)
A description of an event or system that is considered to be accurate.
(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.]
(uncountable) The underlying principles or methods of a given technical skill, art etc., as opposed to its practice. [from 17th c.]
(mathematics) A field of study attempting to exhaustively describe a particular class of constructs. [from 18th c.]
A hypothesis or conjecture. [from 18th c.]
(countable, logic) 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).
(obsolete) Mental conception; reflection, consideration. [16th–18th c.]
Examples
As they encrease the hatred of vice in some, so doe they enlarge the theory of wickednesse in all.
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".
Some of the most beautiful and thus appealing physical theories, including quantum electrodynamics and quantum gravity, have been dogged for decades by infinities that erupt when theorists try to prod their calculations into new domains. Getting rid of these nagging infinities has probably occupied far more effort than was spent in originating the theories.
Lopukhov wrote a number of books and articles on ballet theory, as well as his memoirs.
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.
A theory is consistent if it has a model.
Related words
synonyms
See also Thesaurus:supposition
meronyms
(in logic): axioms
holonyms
(in logic): formal system
related terms
theorem
theoretical
theorise, theorize