I. (ˈ)trī|enēəl, -nyəl adjective
Etymology: Latin trienni um period of three years (from tri- + -ennium, from annus year) + English -al — more at annual
1. : continuing or having a term of three years
a triennial reign
a triennial parliament
2. : occurring, appearing, or being made, done, or acted upon every three years
a triennial election
II. noun
( -s )
Etymology: Latin trienni um + English -al, n. suffix
: a triennial event, appearance, or occasion: as
a. : a triennial episcopal visitation in the Church of England
b. : a third anniversary
c. : something (as a serial publication) that appears every three years