The number 21 is not a prime number. Prime numbers are numbers greater than one that are evenly divisible by only 1 and themselves. As the number 21 has multiple factors, including 1, 21, 3 and 7, it does not meet the definition of a prime number.
The number 21 is what is known as a composite number, which is an integer that is evenly divisible by a positive number other than itself and one. All composite numbers can be broken down into two or more prime factors. Since 7 x 3 = 21, and 7 and 3 are both prime numbers, they are the prime factors of 21. Semiprime numbers are numbers that have only two prime factors, so 21 is a semiprime number.