A prime number (or a prime) is a natural number greater than 1 that has no positive divisors other than 1 and itself. The number 3, for example, is a prime number. Now the update: The Great Internet Mersenne Prime Search project has announced the discovery of the largest known prime. It is 2 times 2 times 2 … (repeat this step 74,207,281 times in all) … minus 1, or (2)^74,207,281 – 1 in formal mathematical notation. (By the way, in case you’re wondering, the next largest prime is (2)^57,885,161 – 1.) Both of these numbers are so-called Mersenne primes, or prime numbers that take the form 2^p – 1, where p is also a prime. There are only 49 known Mersenne primes. (Read more about this mathematical discovery here.)