A prime number is a positive integer that can be divided evenly by only two numbers: 1 and itself. Thus, prime numbers only have two distinct natural factors. For example, 2, 3, 5, 7, 9, 11, 13, 17, and 19 are all prime numbers. 4, 6, 9, 12, 14 are NOT prime numbers. The number 2 is both the smallest and the only even prime number. Neither 0 nor 1 is a prime number and all prime numbers are positive. 1 is not a prime number because it has only one divisor. A number that is not a prime number is called a composite number.

Using symbols, a number n > 1 is prime if it cannot be written as a product of two factors a and b, both of which are larger than 1: n = a "¢ b.