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

The Collaborative International Dictionary of English v.0.48:

Atomic \A*tom"ic\, Atomical \A*tom"ic*al\, a. [Cf. F. atomique.]
   1. Of or pertaining to atoms.
      [1913 Webster]

   2. Extremely minute; tiny.
      [1913 Webster]

   Atomic bomb, see atom bomb in the vocabulary.

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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, atomic
       theory, atomist theory, atomistic theory] [ant:
       holism, holistic theory]