31 is a prime number. This is due to the fact that it is only divisible by itself and the number one, which is the criteria for any prime number.
For a number to be classified as a prime number, it has to be greater than one and be a whole number. 31 is the 11th prime number in the list of prime numbers when beginning from the number 2, which is the first prime number. There are no other numbers apart from 31 and 1 that divide equally into 31, which makes it a prime number. Incidentally, the number one does not count as a prime number.