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.

Atomic \A*tom"ic\, Atomical \A*tom"ic*al\, a. [Cf. F. atomique.]
   1. Of or pertaining to atoms.

   2. Extremely minute; tiny.

   {Atomic philosophy}, or {Doctrine of atoms}, a system which,
      assuming that atoms are endued with gravity and motion,
      accounted thus for the origin and formation of all things.
      This philosophy was first broached by Leucippus, was
      developed by Democritus, and afterward improved by
      Epicurus, and hence is sometimes denominated the Epicurean
      philosophy.

   {Atomic theory}, or the {Doctrine of definite proportions}
      (Chem.), teaches that chemical combinations take place
      between the supposed ultimate particles or atoms of
      bodies, in some simple ratio, as of one to one, two to
      three, or some other, always expressible in whole numbers.
      

   {Atomic weight} (Chem.), the weight of the atom of an element
      as compared with the weight of the atom of hydrogen, taken
      as a standard.

Source : WordNet®

atomic theory
     n 1: a theory of the structure of the atom
     2: (chemistry) any theory in which all matter is composed of
        tiny discrete finite indivisible indestructible particles;
        "the ancient Greek philosophers Democritus and Epicurus
        held atomic theories of the universe" [syn: {atomism}, {atomist
        theory}, {atomistic theory}] [ant: {holism}]