The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   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.
         [1913 Webster]

   2. An exposition of the general or abstract principles of any
      science; as, the theory of music.
      [1913 Webster]

   3. The science, as distinguished from the art; as, the theory
      and practice of medicine.
      [1913 Webster]

   4. The philosophical explanation of phenomena, either
      physical or moral; as, Lavoisier's theory of combustion;
      Adam Smith's theory of moral sentiments.
      [1913 Webster]

   Atomic theory, Binary theory, etc. See under Atomic,
      Binary, etc.
      [1913 Webster]

   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.
          [1913 Webster] Theosoph

WordNet (r) 3.0 (2006):

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 insight into 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,
       theory]
    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"

Moby Thesaurus II by Grady Ward, 1.0:

92 Moby Thesaurus words for "theory":
   apprehension, assumption, attitude, base, basis, body of theory,
   climate of opinion, common belief, community sentiment, conceit,
   concept, conception, conclusion, conjecture, consensus gentium,
   consideration, estimate, estimation, ethos, explanation, eye,
   fancy, feeling, general belief, grounds, guess, guesswork,
   harmonics, harmony, hunch, hypothesis, idea, image, imago,
   impression, intellectual object, judgment, lights, melodics,
   memory-trace, mental image, mental impression, mind, music,
   music theory, musicality, musicography, musicology, mystique,
   notion, observation, opinion, perception, perhaps,
   personal judgment, point of view, popular belief, position,
   posture, premise, presentiment, presumption, prevailing belief,
   public belief, public opinion, reaction, recept, reflection,
   representation, rhythmics, sentiment, sight, speculation, stance,
   supposal, suppose, supposition, surmise, suspicion,
   tentative explanation, theoretical structure, there, thinking,
   thither, thitherward, thought, understanding, unified theory, view,
   way of thinking, yon, yonder



The Jargon File (version 4.4.7, 29 Dec 2003):

theory
 n.

    The consensus, idea, plan, story, or set of rules that is currently being
    used to inform a behavior. This usage is a generalization 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....?


The Free On-line Dictionary of Computing (30 December 2018):

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)