An integer $n \in \Z$ is even if and only if it is of the form:
where $r \in \Z$ is an integer.
The concept of classifying numbers as odd or even appears to have originated with the Pythagoreans.
It was their belief that even numbers are female, and odd numbers are male.
A commentator on Plato used the term isosceles number for an even number, in correspondence with the concept of an isosceles triangle. In a similar way an odd number was described as scalene.