A whole number that is only divided evenly between 1 and itself. For example, 2, 3, 5, 7, 11, 13, 17, 19, and 23 are all examples of prime numbers. Prime numbers are often used in cryptography and as a method for generating random numbers.
For example, with RSA encryption, two prime numbers are multiplied by each other to generate a semiprime, which creates a public key. For someone to decrypt the message, they would need to determine the two prime numbers used to create the semiprime. With large prime numbers, it is extremely complex and time intensive to determine those two numbers.
Those interested in prime numbers and want to help with discovering new prime numbers may want to consider the distributed computing program GIMPS (Great Internet Mersenne Prime Search). GIMPS helped discover the largest known prime number containing 17,425,170 digits on January 25, 2013.