From Late Latin theōria, from Ancient Greek θεωρία (theōria, “contemplation, speculation, a looking at, things looked at”), from θεωρέω (theōreō, “I look at, view, consider, examine”), from θεωρός (theōros, “spectator”), from θέα (thea, “a view”) + ὁράω (horaō, “I see,look”).
theory (countable and uncountable; plural theories)
- (obsolete) Mental conception; reflection, consideration. [16th-18th c.]
- 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.
- (sciences) A coherent statement or set of ideas that explains observed facts or phenomena, or which sets out the laws and principles of something known or observed; a hypothesis confirmed by observation, experiment etc. [from 17th c.]
- 2002, Duncan Steel, The Guardian, 23 May 2002:
- 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.
- 2003, Bill Bryson, A Short History of Nearly Everything, BCA, p. 118:
- The world would need additional decades [...] before the Big Bang would begin to move from interesting idea to established theory.
- 2009, Richard Dawkins, The Greatest Show On Earth: The Evidence for Evolution, Bantam, p. 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".
- 2012 January 1, Michael Riordan, “Tackling Infinity”, American Scientist, volume 100, number 1, page 86:
- 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.
- (uncountable) The underlying principles or methods of a given technical skill, art etc., as opposed to its practice. [from 17th c.]
- 1990, Tony Bennett, Outside Literature, p. 139:
- Does this mean, then, that there can be no such thing as a theory of literature?
- 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.
- (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.
- 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?
- 2003, Sean Coughlan, The Guardian, 21 Jun 2003:
- The theory is that by stripping costs to the bone, they are able to offer ludicrously low fares.
- (countable) (logic) A set of axioms together with all statements derivable from them. Equivalently, a formal language plus a set of axioms (from which can then be derived theorems).
- A theory is consistent if it has a model.
Usage notes 
In scientific discourse, the sense “unproven conjecture” is discouraged (with hypothesis or conjecture preferred), due to unintentional ambiguity and intentional equivocation with the sense “well-developed statement or structure”.
Derived terms 
terms derived from theory
Related terms 
a coherent set of statements attempting to explain observed phenomena
a field of study in mathematics
in logic: a set of axioms and all statements derivable from them
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Translations to be checked
See also