mentally and physically moving onto another person they both go hand in hand though
if you can't mentally stay with a person, you can't physically stay with them; if you're out of it physically, then you're out of the relationship mentally as well
Any kind of sexual contact. This includes kissing. And alcohol and drugs are no excuse. Talking to somebody else in a non-friendship kind of way can also be considered cheating, but is harder to prove unless there are photos or explicit texts.
I also believe in 'mental' cheating, but that's different and I think that we all do it at some point so it's excusable.