The Collaborative International Dictionary of English v.0.48:

Positivism \Pos"i*tiv*ism\, n.
   A system of philosophy originated by M. Auguste Comte, which
   deals only with positives. It excludes from philosophy
   everything but the natural phenomena or properties of
   knowable things, together with their invariable relations of
   coexistence and succession, as occurring in time and space.
   Such relations are denominated laws, which are to be
   discovered by observation, experiment, and comparison. This
   philosophy holds all inquiry into causes, both efficient and
   final, to be useless and unprofitable.
   [1913 Webster]

WordNet (r) 3.0 (2006):

positivism
    n 1: the form of empiricism that bases all knowledge on
         perceptual experience (not on intuition or revelation)
         [syn: positivism, logical positivism]
    2: a quality or state characterized by certainty or acceptance
       or affirmation and dogmatic assertiveness [syn: positivity,
       positiveness, positivism] [ant: negativeness,
       negativism, negativity]

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "positivism":
   Marxism, animalism, atomism, behaviorism, bigotry,
   commonsense realism, dialectical materialism, dogmaticalness,
   dogmatism, down-to-earthness, earthiness, earthliness, empiricism,
   epiphenomenalism, freedom from illusion, hardheadedness,
   historical materialism, hylomorphism, hylotheism, hylozoism,
   infallibilism, lack of feelings, materialism, matter-of-factness,
   mechanism, natural realism, naturalism, new realism,
   opinionatedness, peremptoriness, physicalism, physicism,
   positive philosophy, positiveness, practical-mindedness,
   practicality, practicalness, pragmaticism, pragmatism, rationality,
   realism, reasonableness, representative realism, saneness,
   scientism, secularism, self-opinionatedness, sensibleness,
   sober-mindedness, substantialism, temporality, unidealism,
   unromanticalness, unsentimentality, worldliness



The Devil's Dictionary (1881-1906):

POSITIVISM, n.  A philosophy that denies our knowledge of the Real and
affirms our ignorance of the Apparent.  Its longest exponent is Comte,
its broadest Mill and its thickest Spencer.