im wondering.. where do i find the declaration of this variable? i think ive searched the whole folder but i dont find any declaration of this variable?

Why do you need to find that at all? You can set the webpath and the serverpath in the zp-data/zp-config.php if needed on your server.

But if you really want to find it, it is in functions-basic.php. Probably you need to get a better searching tool, it is not all that hard to locate. But as acrylian says, you probably should not be changing that define. Things are likely to fail should you do so.