cheater; unfaithful person; adulterer
Refers to a person who is unfaithful in a romantic relationship; can be used for either gender, though often for men. Slightly colloquial.
He's a cheater, so his girlfriend dumped him.
You can't trust a cheater.
Compound of 浮気 (うわき, 'unfaithfulness') and 者 (もの, 'person'). The exact origin of 浮気 is uncertain, but it is a native Japanese word.