A number is a member of a group of other items for which one or more opertations (like addition or multiplication) is defined that result in other members of the group. For example:

Counting numbers: 1, 2, 3, ...
Whole numbers: 1, 2, 3, ... and sometimes 0, 1, 2, 3, ...
Integers: -2, -1, 0, 1, 2, ...
Rational numbers: 1/2, -3/7, 0, ...

Zero is not a counting number, but it is integer, rational, real, complex number and many others.

The notion of evenness applies to integers. A number N is even when it's evenly divisible by 2, which means that

N = N/2 · 2.

Therefore, by definition, zero is an even number.