The Collaborative International Dictionary of English v.0.48:

Trinitarianism \Trin`i*ta"ri*an*ism\, n.
   The doctrine of the Trinity; the doctrine that there are
   three distinct persons in the Godhead.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Trinitarianism
    n 1: Christian doctrine stressing belief in the Trinity