The Collaborative International Dictionary
Trieterical
Trieterical \Tri`e*ter"ic*al\, a. [L. trietericus, Gr. ?, fr. ?
(sc. ?) a triennial festival; ? (see Tri-.) + ? a year.]
Kept or occurring once in three years; triennial. [R.]
--J.
Gregory.
Wiktionary
trieterical
a. (context rare English) triennial: occurring once in three years.