## What is a prime number?

**Prime** numbers (or primes) are natural numbers greater than 1 and divisible only by themselves and by the number 1. For example, 5 is prime because it can only be divided by 1 and itself. Number theory is founded on the central role of primes, established by the fundamental theorem of arithmetic: A number greater than 1 is either a prime or can be expressed as a prime product that is unique. Modern cryptography uses prime numbers to encrypt data. The higher the prime number, the more secure the cryptography is since the factorization process takes so much time.