vicennial

adj

1. (not-comparable)Occurring once every twenty years.
2. (not-comparable)Lasting for twenty years.

noun

1. A twenty-year period

   “The maps are presented at 20-year intervals (vicennials) because of the nature of the data, particularly for years prior to 1960 when estimation procedures were not nearly as developed as today.”

2. A twentieth anniversary, particularly

   “Among the well established anniversaries celebrated by Chautauqua classes from year to year are the vicennials. A strong sense of comradeship is developed among members who come to Chautauqua twenty years after graduation...”

3. (historical)A twentieth anniversary, particularly

   “The feast of Constantine's vicennials fell on the 25th of July this year: it was the commencement of the twentieth year of his reign. It is supposed, that to avoid interrupting affairs or greater moment, this ceremony was deferred till the end of the council, which broke up the 25th of August.”

   “Roman coins mentioning the vota publica for Macrinus are RIC IV.1 5–13; on vota publica for the decennials and vicennials of emperors in the 2nd and 3rd c. ad, see Chastagnol 1984.”