Webster 1913 Edition
Continuing three years;
Happening, coming about, or appearing once in every three years;
Something which takes place or appears once in three years.
Webster 1828 Edition
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.