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.