15

Fifteen (15) is a positive integer one more than 14 and one less than 16.

Properties
15 is the second odd composite number, being 3*5. It is also a semiprime.

Because 15's factorization only contains Fermat primes repeated once, a regular 15-sided polygon is constructible with compass and straightedge.

15 is the fifth triangular number (between 10 and 21), and the third pentatope number (between 5 and 35).

15 is a lucky number, between 13 and 21.