Sixty-three is not a prime number. Sixty-three is classified as a composite number because it has divisors, and can be composed of other numbers than number one.
Numbers are identified as either prime or composite numbers, with the exception of one. To be classified as a prime number, a number must be greater than one and only divisible by one and itself.
Sixty-three, however, is not a prime number. It is divisible by the following multiple numbers: 1, 3, 9, 7 and 21.
All composite numbers, like 63, have prime divisors. The prime divisors of 63 are 3 and 7.