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The preposition 'on' is very important in this context.
To cheat on someone means to secretly have sex with someone other than your husband, wife, or partner. You can also say that someone is unfaithful to their husband, wife, or partner. In this video she discovered he'd been cheating on her and slapped him and then shoved him
To cheat means to act dishonestly or unfairly in order to gain an advantage.
A cheat is a person that cheats.