It has more than two factors.
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Composite
number 21 is prime
21 is a composite number, not a prime, having prime factors 3x7.
21 is the product of two prime numbers, three and seven. A prime number is defined as a number divisible only by one or itself. So ... NO ... 21 is not a prime number.
21 is composite number that is a multiple of 3. 31 is a prime number.