The Uncultured Anonymous/Cheating

From Bibliotheca Anonoma

Cheating involves purposefully skirting the established rules of conduct for people. It often involves other methods of deceit such as lying. For example, you might lie if someone asks you if you were cheating.

Cheating also commonly refers to dating or fucking someone 'behind the back' of someone else you are seeing, who would not want you to be seeing that person or at least would want to know about it. When trust is established and there is an agreement to be open about such matters, not revealing them is deceitful and termed cheating. This is most common in monoamorous relationships.