triennial /tɹaɪˈɛni.əl/adjective 2noun 2forms 1 adjective2 •Happening every three years.They were constantly summoned to a triennial festival : and the most approved songs delivered at this assembly were ordered to be preserved in the custody of the king's historian or antiquary — 1774 •Lasting for three years. noun2 •A third anniversary. •A plant that requires three years to complete its life-cycle. formstriennials
