Lewis & Short

vīcennālis, e, adj. [vicennium], of twenty years, every twenty years (postclass.).

  1. I. Adj.: votis vicennalibus, Num. Alex. Sev. ap. Eckhel. D. N. V. 7, p. 275.
  2. II. Subst.: vīcennālĭa, ĭum, n., a festival on the twentieth anniversary of an emperor’s reign, Lact. Mort. Pers. 17.