TRIEN'NIAL, adjective [Latin triennis, triennium; tres, three, and annus, year.] 1. Continuing three years; as triennial parliaments. 2. Happening every three years; as triennial elections. triennial elections and parliaments were established in England in 1695; but these were discontinued in 1717, and septennial elections and parliaments were adopted, which still continue.
Source: Webster's 1828 American Dictionary ↗ · public-domain · returned verbatim, no AI