theory
Pronunciation
  • (Received Pronunciation, General Australian) IPA: /ˈθɪəɹi/, /ˈθiːəɹi/, /ˈθɪɹi/
  • (General American) IPA: /ˈθiːəɹi/, /ˈθɪɹi/
Noun

theory

  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.]
    • 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".
  4. (uncountable) The underlying principles or methods of a given technical skill, art etc., as opposed to its practice. [from 17th c.]
  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.]
    • 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 Deductive closure. 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.
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