theory

Etymology

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

1. 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. 1646, Thomas Browne, Pseudodoxia Epidemica, VII.19
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.
   In its most proper acceptation, theory means the completed result of philosophical induction from experience. 1843, John Stuart Mill, ""A System of Logic, Ratiocinative and Inductive, ..., Bk V, Ch 7
   Does this mean, then, that there can be no such thing as a theory of literature? 1990, Tony Bennett, Outside Literature, page 139
   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. 23 May 2002, Duncan Steel, The Guardian
   The world would need additional decades … before the Big Bang would begin to move from interesting idea to established theory. 2003, Bill Bryson, A Short History of Nearly Everything, BCA, page 118
   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". 2009, Richard Dawkins, The Greatest Show On Earth: The Evidence for Evolution, Bantam, page 10
   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. 2012-01, Michael Riordan, “Tackling Infinity”, in American Scientist, volume 100, number 1, archived from the original on 2013-04-30, page 86
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. 1998, Elizabeth Souritz, The Great History of Russian Ballet
4. (mathematics) A field of study attempting to exhaustively describe a particular class of constructs.
   Knot theory classifies the mappings of a circle into 3-space.
5. A hypothesis or conjecture.
   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? 1999, Wes DeMott, Vapors
   The theory is that by stripping costs to the bone, they are able to offer ludicrously low fares. 21 Jun 2003, Sean Coughlan, The Guardian
6. (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).
   A theory is consistent if it has a model.
7. (obsolete) Mental conception; reflection, consideration.