资料来源 : Webster's Revised Unabridged Dictionary (1913)

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.