\Tri*en"ni*al\, a. [L. triennium the space of three
years; tri- (see {Tri-}) + annus year. See {Annual}.]
1. Continuing three years; as, triennial parliaments; a
   triennial reign. --Howell.
2. Happening, coming about, or appearing once in every three
   years; as, triennial elections; a triennial catalogue; a
   triennial visitation. --T. Warton.

\Tri*en"ni*al\, n.
Something which takes place or appears once in three years.