Source : Webster's Revised Unabridged Dictionary (1913)

Triune \Tri"une\, a. [Pref. tri- + L. unus one. See {One}.]
   Being three in one; -- an epithet used to express the unity
   of a trinity of persons in the Godhead.

Source : WordNet®

triune
     adj : being three in one; used especially of the Christian
           Trinity; "a triune God"