A prime number is a whole number with exactly two integral divisors, 1 and itself. Any number greater than 2 which is a multiple of 2 is not a prime, since it has at least three divisors 1, 2 and itself. Any number greater than 3 which is a multiple of 3 is not a prime, since it has 1and 3 as divisors. 43 is a prime number because it has only two distinct divisors: 1 and itself (43).
