1. Occurring once every 15 years.
2. Lasting 15 years.
A 15th anniversary.
[From Latin quīndecim, fifteen; see penkwe in Indo-European roots + Latin -ennium (from annus, year; see at- in Indo-European roots).]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
occurring once every 15 years or over a period of 15 years
a 15th anniversary
[C20: from Latin quindecim fifteen + annus year, on the model of biennial]
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014