What is "vicennial"

Vicennial \Vi*cen"ni*al\, a. [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.

a. 1 occurring once every twenty years. 2 lasting for twenty years.

adj. occurring once every 20 years