Undecennial
Pronunciation : Un`de*cen"ni*al
Part of Speech : a.
Etymology : [See Undecennary, and cf. Decennial.]
Definition : Defn: Occurring or observed every eleventh year; belonging to, or continuing, a period of eleven years; undecennary; as, an undecennial festival.
Source : Webster's Unabridged Dictionary, 1913