An integer which can be divided by 2; the series of every other integer beginning with 2 (or 0).
The ancient Greeks are purported to have created the distinction between even and odd numbers because of their musical studies:
Because the 2:1 frequency ratio, or octave, gives a sound which so strongly resembles the 1:1 or starting pitch, the Greeks characterized this 2:1 relationship as female and even.
3:1, on the other hand, created an entirely new pitch, and so (along with 1:1) was considered to be male and odd.
4:1 again produces another 'octave' of the original pitch, and so is again female and even.
5:1 again produces another new pitch, and so is male and odd.
6:1 produces a pitch an 'octave' higher than 3:1, and so is again female and even.
...and so on.
It is very easy to tell even numbers from odd: any number whose final digit is divisible by 2, is an even number.