Forty-three is a prime number. This is due to the fact that it can only be divided by itself and the number one, which is how a number qualifies as a prime number.
For a number to be a prime number, it can not be divisible wholly by any number other than itself and one. As there are no other numbers that can be completely divided into the number 43 without anything being left over, that makes 43 a member of the set of prime numbers. The list of prime numbers begins with the number two, and 43 is the 13th prime number.