The Collaborative International Dictionary
Undecennial
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.
Douglas Harper's Etymology Dictionary
undecennial
"occurring every 11 years," 1858, in reference to solar activity cycle, from Latin undecim "eleven" + ending from biennial, etc.
Wiktionary
undecennial
a. Occurring or observed every eleventh year.