adulterous partner; lover in an affair
Refers to the person one is having an extramarital or otherwise unfaithful relationship with. The term is neutral in register but carries a strong negative moral connotation.
He often lies to meet his lover.
The wife went to see her husband's mistress directly.
浮気 refers to the act of cheating or being unfaithful, while 浮気相手 specifically denotes the person one is cheating with.
不倫相手 is a more formal or serious term for an extramarital affair partner, often used in legal or dramatic contexts. 浮気相手 is more colloquial and can refer to any unfaithful relationship, not necessarily marital.
Compound of 浮気 (うわき, 'unfaithfulness, extramarital affair') and 相手 (あいて, 'partner, companion').