
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. 