An even number is a number that can be divided into two equal groups.
An odd number is a number that cannot be divided into two equal groups.
Even numbers end in 2, 4, 6, 8 and 0 regardless of how big they are, how many digits they have (we know the number 5,917,624 is even because it ends in a 4).
Odd numbers end in 1, 3, 5, 7, 9.