The Collaborative International Dictionary of English v.0.48:

Dualism \Du"al*ism\, n. [Cf. F. dualisme.]
   State of being dual or twofold; a twofold division; any
   system which is founded on a double principle, or a twofold
   distinction; as:
   (a) (Philos.) A view of man as constituted of two original
       and independent elements, as matter and spirit. (Theol.)
   (b) A system which accepts two gods, or two original
       principles, one good and the other evil.
   (c) The doctrine that all mankind are divided by the
       arbitrary decree of God, and in his eternal
       foreknowledge, into two classes, the elect and the
       reprobate.
   (d) (Physiol.) The theory that each cerebral hemisphere acts
       independently of the other.
       [1913 Webster]

             An inevitable dualism bisects nature, so that each
             thing is a half, and suggests another thing to make
             it whole.                            --Emerson.
       [1913 Webster]

WordNet (r) 3.0 (2006):

dualism
    n 1: the doctrine that reality consists of two basic opposing
         elements, often taken to be mind and matter (or mind and
         body), or good and evil

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "dualism":
   Janus, acosmism, allotheism, ambiguity, ambivalence, anthropolatry,
   anthropomorphism, anthropotheism, autotheism, biformity,
   bifurcation, conjugation, cosmotheism, deism, dichotomy, ditheism,
   doubleness, doublethink, doubling, duality, duplexity, duplication,
   duplicity, dyotheism, equivocality, halving, henotheism,
   hylotheism, irony, monolatry, monotheism, multitheism, myriotheism,
   pairing, pantheism, physicomorphism, physitheism, pluralism,
   polarity, polytheism, psychotheism, tetratheism, theism,
   theopantism, theriotheism, tritheism, twinning, two-facedness,
   twoness, zootheism