A "prime factor" of a number is a factor of the number which also
happens to be a prime number. So, 2 and 3 are prime factors of 18 but
the others are not. For instance, 6 is a factor of 18 but it is not a
prime factor of 18 because 6 is a composite number. I should add that
there is not just one prime factor of 18, because there are two of
them.