Is ZERO an integer?

Answer 1

Sure!
There is no problem to initialize a variable like this:

int variable = 0;

Don't make confusions between 0 (zero) and NULL, or '\0' (these are in C) or nil (Swift) and many more.

But the conclusion is that zero is an integer.

In programming there are some discussions on -0 and +0. Just read about the one's and two's complement in programming.