quindecennial 1. Happening once every fifteen years or a fifteenth anniversary. 2. Lasting for fifteen years. 3. A fifteenth year of existence or duration; fifteenth anniversary. From Latin quindecim, 'fifteen' [quinque, 'five' plus decem, 'ten' plus -ennial, 'year'].
of or pertaining to a period of 15 years or the 15th occurrence of a series, as an anniversary.