The Collaborative International Dictionary of English v.0.48:

Naturalism \Nat"u*ral*ism\, n. [Cf. F. naturalisme.]
   1. A state of nature; conformity to nature.
      [1913 Webster]

   2. (Metaph.) The doctrine of those who deny a supernatural
      agency in the miracles and revelations recorded in the
      Bible, and in spiritual influences; also, any system of
      philosophy which refers the phenomena of nature to a blind
      force or forces acting necessarily or according to fixed
      laws, excluding origination or direction by one
      intelligent will.
      [1913 Webster]

   3. The theory that art or literature should conform to
      nature; realism; also, the quality, rendering, or
      expression of art or literature executed according to this
      theory.
      [Webster 1913 Suppl.]

   4. Specifically: The principles and characteristics professed
      or represented by a 19th-century school of realistic
      writers, notably by Zola and Maupassant, who aimed to give
      a literal transcription of reality, and laid special
      stress on the analytic study of character, and on the
      scientific and experimental nature of their observation of
      life.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

naturalism
    n 1: (philosophy) the doctrine that the world can be understood
         in scientific terms without recourse to spiritual or
         supernatural explanations
    2: an artistic movement in 19th century France; artists and
       writers strove for detailed realistic and factual description
       [syn: naturalism, realism]

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "naturalism":
   Marxism, absolute realism, animalism, artlessness, atomism,
   authenticity, behaviorism, bona fideness, commonsense realism,
   dialectical materialism, earthliness, empiricism, epiphenomenalism,
   genuineness, health, historical materialism, honesty, hylomorphism,
   hylotheism, hylozoism, inartificiality, legitimacy, lifelikeness,
   literalism, literality, literalness, materialism, mechanism,
   natural realism, naturalness, nature, naturism, new realism,
   normalcy, normality, normalness, order, photographic realism,
   physicalism, physicism, positive philosophy, positivism,
   pragmaticism, pragmatism, propriety, realism, realness, regularity,
   representative realism, secularism, sincerity, substantialism,
   temporality, true-to-lifeness, truth to nature, unadulteration,
   unaffectation, unaffectedness, unartificialness, unassumingness,
   undisguise, unfictitiousness, unpretentiousness, unspeciousness,
   unspoiledness, unspuriousness, unsyntheticness, verisimilitude,
   wholesomeness, worldliness