Question

if a # is not a prime # what is it?

Answer 1

If a positive integer is not a prime, it is either 1 or a composite number.

1 is considered neither a prime nor a composite number.

The question is:
"if a # is not a prime # what is it?"

A number can be real, complex, positive, negative, or anything else. So the answer depends on to which set the "number" belongs.