Cheating means breaking the agreements that your relationships are conditional upon.
That's more than just monogamy agreements; if someone agrees not to enter into new relationships without another partner's knowledge, and then does so, that's still cheating. If you have a "condom commitment" with one of your partners and then have *unprotected* sex with someone else, that's cheating too.
Hear hear! And you must know my ex! I still have to explain why I can call it cheating that he had sex with our roommate after agreeing not to, even tho we're poly. So many times, even poly people don't understand when I say, "When you make a promise and then break it, that's cheating." I don't care if it's cheating at cards (agreeing to play by the rules set forth and then not) or in relationships (which I guess is the same definition).
Brigitte