Source : Webster's Revised Unabridged Dictionary (1913)

Theory \The"o*ry\, n.; pl. {Theories}. [F. th['e]orie, L.
   theoria, Gr. ? a beholding, spectacle, contemplation,
   speculation, fr. ? a spectator, ? to see, view. See
   {Theater}.]
   1. A doctrine, or scheme of things, which terminates in
      speculation or contemplation, without a view to practice;
      hypothesis; speculation.

   Note: ``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.

   2. An exposition of the general or abstract principles of any
      science; as, the theory of music.

   3. The science, as distinguished from the art; as, the theory
      and practice of medicine.

   4. The philosophical explanation of phenomena, either
      physical or moral; as, Lavoisier's theory of combustion;
      Adam Smith's theory of moral sentiments.

   {Atomic theory}, {Binary theory}, etc. See under {Atomic},
      {Binary}, etc.

   Syn: Hypothesis, speculation.

   Usage: {Theory}, {Hypothesis}. 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.