Definitions for vicennialvaɪˈsɛn i əl
Random House Webster's College Dictionary
vi•cen•ni•alvaɪˈsɛn i əl(adj.)
of or for twenty years.
occurring every twenty years.
Origin of vicennial:
1730–40; < L
occurring once every 20 years
Occurring every twenty years.
Lasting twenty years.
lasting or comprising twenty years
happening once in twenty years; as, a vicennial celebration