No it's not just what you're paid for, I've already clarified this for two other commenters.
Grab a dictionary and you'll see something along "activity involving mental or physical effort done in order to achieve a purpose or result".
Unless your side projects have no purpose or goals, they are a form of work and they require effort.
Work is what I am paid for. Anything else is either your human needs or hobbies.
I prefer a colleague's response to my question whether he has side projects: I don't like to do the same as I do (at work) here when I am home.
It's an incorrect answer, but at least it doesn't imply side projects are work.