Cheating IMO is doing something (be it emotional or physical) with someone else which you know your partner would disapprove of, sometimes strongly. Its all in the eye of the beholder really, if your partner considers it cheating, then its cheating.
Where it gets complicated is when you two aren't exclusive but the same rules apply. If I'm in a serious poly relationship of sorts with someone, that involves doing things that would normally be considered cheating my most couples standards. A good solid base of communication really is key here as in any relationship. Still, if my partner is doing something that I would disapprove of or without my consent I would certainly consider that cheating. Boundaries are important in any sort of relationship.
Sorry, last paragraph went off on a bit of a tangent but its something I've been thinking about a lot lately...