Adultery is when a married person has illicit sexual relations with someone who is not that person's spouse. It is a breaking of the marriage covenant and is a great sin. Adultery is forbidden in the 10 Commandments (Exo. 20:14, Deut. 5:18, Lev. 18:20). In the Old Testament, it was punishable by death (Deut. 22:22-24) and was intended to be carried out in the case of the adulterous woman in John 7:53-8:11.
In the New Testament, it is mentioned in numerous verses such as Matt. 5:27-28, 32, 19:18, Luke 16:18, Rom. 2:22, 1 Cor. 6:9, Heb. 13:4, etc.