Anything outside the rules set by those in the relationship. Maybe you have agreed that porn is off-limits. If you agreed to that, then watch it, that is cheating, by the rules of that relationship. On the other hand, maybe you’re swingers, and the only limit is no kissing other people on the lips. So you can have sex with another couple and it’s not cheating. But if you kiss another couple while having sex, that would be cheating, by the rules of that relationship.