Source : Webster's Revised Unabridged Dictionary (1913)

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.