Fifteen is not a prime number. A prime number is a positive number that has only two possible divisors, 1 and itself. Fifteen can be divided by 3 and 5 as well as 1 and itself.
Fifteen is a composite number. Composite numbers are positive, natural numbers that have at least one other number that divides into it besides itself and 1. An example of a prime number is 2. This number does not have any other divisors besides 1 and itself. Some other prime numbers are 5, 7 and 11.
Defining whether a number is prime or not is known as the property of primality.