the fact of having a sexual relationship or experience with a person who is not your husband, wife, or usual sexual partner

His sister accused their father of adultery and unfaithfulness. 他的姐姐指责他们的父亲与人通奸，对婚姻不忠。

Many people discover that their partner's unfaithfulness leaves them feeling deeply betrayed. 许多人发现，伴侣的不忠让他们深感被背叛。