Everyone has different ideas on what is considered cheating. What are your ideas?

Cheating is defined as an act of lying, deception, fraud, trickery, imposture, or imposition.

I personally think cheating is any violation of the mutually agreed-upon rules or boundaries of a relationship.

If you are doing something that you would not do in front of your partner, then you're probably cheating.

If you feel that you would be upset, angry, or jealous with these behaviors from your partner, chances are you're doing something that isn't quite okay (this includes flirting, any sort of unnecessary physical contact, talking/texting another person of the opposite sex without your partner's knowledge).

ERROR: You must spread some Reputation around before giving it to Devorameira again.

As Devorameira said, whatever you wouldn't do in front of your partner is cheating.

It's just my opinion but I include fantasies to that. If you think of someone else while you kiss your partner, it means to me that something went wrong somewhere.

To my mind if you can't own up to doing something don't do it. Our partners might not always like everything we do but if we can look them in the eye and feel OK about it then that's their problem, or something that can be discussed and compromise agreed on openly. If we can't then we are cheating. That's my take on it.