The Collaborative International Dictionary of English v.0.48:

Theism \The"ism\, n. [From Gr. ? God; probably akin to ? to pray
   for, ? spoken by God, decreed: cf. F. th['e]isme. Cf.
   Enthusiasm, Pantheon, Theology.]
   The belief or acknowledgment of the existence of a God, as
   opposed to atheism, pantheism, or polytheism.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Theism \The"ism\ (th[=e]"[i^]z'm), n. [NL. & E. thea tea +
   -ism.] (Med.)
   The morbid condition resulting from the excessive use of tea.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

theism
    n 1: the doctrine or belief in the existence of a God or gods
         [ant: atheism, godlessness]

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "theism":
   acosmism, adoration, allotheism, anthropolatry, anthropomorphism,
   anthropotheism, autotheism, churchgoing, conformity, cosmotheism,
   cultism, deism, devotedness, devotion, devoutness, ditheism,
   dualism, dutifulness, dyotheism, faith, faithfulness, henotheism,
   hylotheism, love of God, monolatry, monotheism, multitheism,
   myriotheism, observance, pantheism, physicomorphism, physitheism,
   pietism, piety, piousness, polytheism, psychotheism, religion,
   religionism, religiousness, reverence, tetratheism, theopantism,
   theriotheism, tritheism, veneration, worship, worshipfulness,
   zootheism