The Collaborative International Dictionary of English v.0.48:

Atomism \At"om*ism\, n. [Cf. F. atomisme.]
   The doctrine of atoms. See Atomic philosophy, under
   Atomic.
   [1913 Webster]

WordNet (r) 3.0 (2006):

atomism
    n 1: (psychology) a theory that reduces all mental phenomena to
         simple elements (sensations and feelings) that form complex
         ideas by association
    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]