Webster 1913 Edition



, L.
, Gr. [GREEK] a beholding, spectacle, contemplation, speculation, fr. [GREEK] a spectator, [GREEK] to see, view. See
A doctrine, or scheme of things, which terminates in speculation or contemplation, without a view to practice; hypothesis; speculation.
☞ “This word is employed by English writers in a very loose and improper sense. It is with them usually convertible into hypothesis, and hypothesis is commonly used as another term for conjecture. The terms theory and theoretical are properly used in opposition to the terms practice and practical. In this sense, they were exclusively employed by the ancients; and in this sense, they are almost exclusively employed by the Continental philosophers.”
Sir W. Hamilton.
An exposition of the general or abstract principles of any science;
as, the
of music
The science, as distinguished from the art;
as, the
and practice of medicine
The philosophical explanation of phenomena, either physical or moral;
as, Lavoisier’s
of combustion; Adam Smith's
of moral sentiments.
Atomic theory
Binary theory
etc. See under
, etc.
Syn. – Hypothesis, speculation.
. A theory is a scheme of the relations subsisting between the parts of a systematic whole; an hypothesis is a tentative conjecture respecting a cause of phenomena.

Webster 1828 Edition



[L. theoria; Gr. to see or contemplate.]
Speculation; a doctrine or scheme of things, which terminates in speculation or contemplation, without a view to practice. It is here taken in an unfavorable sense, as implying something visionary.
An exposition of the general principles of any science; as the theory of music.
The science distinguished from the art; as the theory and practice of medicine.
The philosophical explanation of phenomena, either physical or moral; as Lavoisier's theory of combustion; Smith's theory of moral sentiments.
Theory is distinguished from hypothesis thus; a theory is founded on inferences drawn from principles which have been established on independent evidence; a hypothesis is a proposition assumed to account for certain phenomena, and has no other evidence of its truth, than that it affords a satisfactory explanation of those phenomena.

Definition 2023





theory (countable and uncountable, plural theories)

  1. (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.
  2. (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”, in 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.
  3. (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.
  4. (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.
  5. 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.
  6. (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”.


  • See also Wikisaurus:supposition




Related terms


See also