An integer which has two, and only two, integer factors.

Put another way: an integer greater than one which can only be exactly divided by itself and one.

For example:

• 7 will only divide exactly by 1 and 7 (so seven is a prime number)
• 8 will divide by 1, 2, 4 and 8 (so 8 is not a prime number)

Most recently updated at 18:30 (British time) on 07 April 2004