Undecennial a. : Occurring or observed every eleventh year; belonging to, or continuing, a period of eleven years; undecennary; as, an undecennial festival.

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Defination from the web:
Undecennial in the Online Dictionary. Meaning of Undecennial. Pronunciation of Undecennial. Translations of Undecennial. ... Occurring or observed every eleventh year; belonging to, or continuing, a period of eleven years; undecennary; as, an undecennial festival. ...