The person with whom someone is having an illicit affair (typically used for a married person's extramarital partner); lover/mistress.
彼女は夫に不倫相手の存在を知られてしまった。
She was found out by her husband to have an extramarital partner.
週刊誌が有名人の不倫相手の写真を掲載した。
A weekly tabloid published photos of the celebrity's affair partner.
不倫相手との関係が家庭を壊すこともある。
A relationship with an extramarital partner can sometimes destroy a family.
彼は不倫相手と別れる決意を固めた。
He decided to break off his relationship with his extramarital partner.