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.

Source : WordNet®

theory
     n 1: a well-substantiated explanation of some aspect of the
          natural world; an organized system of accepted knowledge
          that applies in a variety of circumstances to explain a
          specific set of phenomena; "theories can incorporate
          facts and laws and tested hypotheses"; "true in fact and
          theory"
     2: a tentative theory about the natural world; a concept that
        is not yet verified but that if true would explain certain
        facts or phenomena; "a scientific hypothesis that survives
        experimental testing becomes a scientific theory"; "he
        proposed a fresh theory of alkalis that later was accepted
        in chemical practices" [syn: {hypothesis}, {possibility}]
     3: a belief that can guide behavior; "the architect has a
        theory that more is less"; "they killed him on the theory
        that dead men tell no tales"

Source : Free On-Line Dictionary of Computing

theory
     
        The consensus, idea, plan, story, or set of rules that is
        currently being used to inform a behaviour.  This usage is a
        generalisation and (deliberate) abuse of the technical
        meaning.  "What's the theory on fixing this TECO loss?"
        "What's the theory on dinner tonight?"  ("Chinatown, I
        guess.")  "What's the current theory on letting lusers on
        during the day?"  "The theory behind this change is to fix the
        following well-known screw...."
     
        (1994-12-14)