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. [ 1913 Webster ]

2. Happening, coming about, or appearing once in every three years; as, triennial elections; a triennial catalogue; a triennial visitation. T. Warton. [ 1913 Webster ]

n. Something which takes place or appears once in three years. [ 1913 Webster ]

adv. Once in three years. [ 1913 Webster ]