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”)].



theory (countable and uncountable, plural theories)

  1. A description of an event or system that is considered to be accurate.
  2. (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.
  3. (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?
    • 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”, in American Scientist[1], 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.
  4. (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.
  5. (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.
  6. 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.
  7. (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.

Usage notesEdit

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”.





Related termsEdit


The translations below need to be checked and inserted above into the appropriate translation tables, removing any numbers. Numbers do not necessarily match those in definitions. See instructions at Wiktionary:Entry layout § Translations.

See alsoEdit


  • theory at OneLook Dictionary Search
  • theory in Keywords for Today: A 21st Century Vocabulary, edited by The Keywords Project, Colin MacCabe, Holly Yanacek, 2018.
  • "theory" in Raymond Williams, Keywords (revised), 1983, Fontana Press, page 316.
  • theory in The Century Dictionary, New York, N.Y.: The Century Co., 1911.