Odd Number

An odd number is a whole number that cannot be divided evenly by two. Formally, an odd number has the form two times k plus one for some whole number k, so numbers such as negative seven, five and twenty-three all count as odd. This sits opposite an even number, one divisible by two and of the form two times k, and zero itself counts as even. Any two whole numbers that follow one after another always have opposite parities, one even and the next odd, and in base ten a number last digit alone shows its parity: it is odd if that digit is one, three, five, seven or nine, and even otherwise. The same idea of parity carries over into binary and other number bases as well. This description is drawn from the Wikipedia article on parity in mathematics, which covers both even and odd numbers together.