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]

Moby Thesaurus II by Grady Ward, 1.0:

114 Moby Thesaurus words for "atomism":
   Aristotelianism, Berkeleianism, Bohr theory, Bradleianism,
   Cynicism, Cyrenaic hedonism, Cyrenaicism, Dirac theory,
   Epicureanism, Fichteanism, Hegelianism, Heideggerianism,
   Heracliteanism, Herbartianism, Humism, Kantianism, Leibnizianism,
   Marxism, Mimamsa, Neo-Hegelianism, Neo-Pythagoreanism,
   Neoplatonism, Peripateticism, Platonism, Purva Mimamsa, Pyrrhonism,
   Pythagoreanism, Sankhya, Schellingism, Scotism, Socratism, Sophism,
   Sophistry, Spencerianism, Stoicism, Thomism, acosmism, agnosticism,
   animalism, animatism, animism, behaviorism, commonsense realism,
   correspondence principle, cosmotheism, criticism, deism,
   dialectical materialism, dualism, earthliness, eclecticism, egoism,
   empiricism, epiphenomenalism, ethics, eudaemonism, existentialism,
   hedonism, historical materialism, humanism, hylomorphism,
   hylotheism, hylozoism, idealism, immaterialism, individualism,
   intuitionism, law of parity, materialism, mechanism, mentalism,
   monism, mysticism, natural realism, naturalism, neocriticism,
   new realism, nominalism, octet theory, ontologism, ontology,
   optimism, organic mechanism, organicism, panpsychism, pantheism,
   pessimism, physicalism, physicism, pluralism, positive philosophy,
   positivism, pragmaticism, pragmatism, psychism,
   psychological hedonism, quantum theory, rationalism, realism,
   representative realism, secular humanism, secularism, semiotic,
   semiotics, sensationalism, skepticism, substantialism, syncretism,
   temporality, theism, transcendentalism, utilitarianism,
   voluntarism, worldliness