Wordnet 3.0
NOUN (1)
1.
(Christianity) the heretical belief that the Father and the Son and the Holy Spirit are three separate gods;
The Collaborative International Dictionary of English v.0.48:
Tritheism \Tri"the*ism\, n. [Pref. tri- + Gr. ? God: cf. F.
trith['e]isme.]
The opinion or doctrine that the Father, Son, and Holy Spirit
are three distinct Gods.
[1913 Webster]
WordNet (r) 3.0 (2006):
tritheism
n 1: (Christianity) the heretical belief that the Father and the
Son and the Holy Spirit are three separate gods