In simple terms, adultery, also known as cheating, infidelity, or having an affair, is the act of being sexually unfaithful in marriage. This is not to ignore cases of emotional unfaithfulness (such as spouses who engage in extramarital "online" affairs) which can have similar devastating effects as the real thing.
Adultery does not simply happen “out of the blue”, although it may seem that way to the betrayed spouse. It is instead the culmination of a long trail of unresolved issues. Similar to an iceberg, the surface above the water is outward and visible, but underneath, there is much more than what is visible to the outside. While an affair is destructive to a relationship, it is a symptom of something much deeper that has been growing for much longer, signifying the end of a painful rope. Counseling at this stage is vital if the relationship is going to be saved. And yes, the good news is that even after something so tragic as infidelity, complete restoration is still possible.