Sixty-one is a prime number. It is the 18th prime number in the series. It is also a centered square number, a centered hexagonal number and a centered decagonal number and the sum of two square numbers.
Prime numbers are whole numbers larger than 1 that can't be divided by any number other than 1 and itself and still result in a whole number. Prime numbers are always odd, because any even number greater than 2 can be divided by 1, 2 and itself. Similarly, any number ending in 5 can't be prime, because it can be divided by 1, 5 and itself.