VICENNIAL

vicennial

(adjective) occurring once every 20 years

Source: WordNet® 3.1

Etymology

Adjective

vicennial (not comparable)

Occurring once every twenty years.

Lasting for twenty years.

Source: Wiktionary

Vi*cen"ni*al, a. Etym: [L. vicennium a period of twenty years; viceni twenty + annus year.]

1. Lasting or comprising twenty years.

2. Happening once in twenty years; as, a vicennial celebration.

Source: Webster’s Unabridged Dictionary 1913 Edition