I think cheating is doing something you wouldn't do if your spouse/mate/significant other was there beside you.
If my husband looks at a beautiful woman when I'm with him, he knows its okay with me. If he goes over and starts kissing her, THATS cheating!
Each couple's set of rules is different. What might be "cheating" for one set wouldn't necessarily be the same for another.
So how can you put a definition on it?