Prime number
A whole number greater than 1 divisible only by 1 and itself.
A prime number is an integer greater than 1 that has no positive divisors other than 1 and itself. Numbers that have additional divisors are composite. Primes are the building blocks of the integers — every whole number factors uniquely into primes — and they are central to cryptography. The number 2 is the only even prime.
Example: 7 is prime; 9 is not, because 9 = 3 × 3.