1) Happening every twenty years 2) Large integer 3) Occurring once every 20 years 
occurring every twenty years
• (a.) Lasting or comprising twenty years. • (a.) Happening once in twenty years; as, a vicennial celebration.
Vi·cen'ni·al adjective [ Latin 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. 
[adj] - occurring once every 20 years
adjective occurring once every 20 years