Anything that you do hoping your partner won't find out.
Breaking promises. Forcing promises which can't be kept. Making promises which you know you'll break. The moment you promise monogamy you're already cheating. Your partner is cheating because s/he forces a promise you'll never be able to keep.<br />
Make promises ba<x>sed on a reality. Make promises you can keep.
Anything that would make your SO unhappy that you did not talk to them about first.
Having cards hidden up your sleeve.!!!
Unless you're having a magic competition. Cheating is breaking of established rules. Those rules differ per couple and per game. It's wise to be clear and honest about the rules you're going to play by.
having emotions for another or physically doing something with another in the process of you dating or being in a relationship with a person
Anything that you wouldn't want to tell your significant other about.
Anything you would only do for or with your boyfriend/girlfriend. with a person who is not.