undecennial


From The Collaborative International Dictionary of English v.0.48:

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]
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