Because 15's factorization only contains Fermat primes repeated once, a regular 15-sided polygon is constructible with compass and straightedge.
15 is a lucky number, between 13 and 21.
15 is equal to 5!!, the double factorial of 5.
In Czech language, 15 is called "mandel".
In googology Edit
In Greek-based number-naming systems, 15 is associated with prefix "pentadeka-", and with prefix "quindec-" in Latin systems.
As a banknote denomination Edit
The third Template:W had banknotes with this number in the denomination.