'Undecennial' definitions:

Definition of 'Undecennial'

From: GCIDE

Undecennial \Un`de*cen"ni*al\, a. [See Undecennary, and cf. Decennial.] Occurring or observed every eleventh year; belonging to, or continuing, a period of eleven years; undecennary; as, an undecennial festival. [1913 Webster]