The unfaithfulness of a married person to the marriage bed; sexual intercourse by a married man with another than his wife, or voluntary sexual intercourse by a married woman with another than her husband. Note: It is adultery on the part of the married wrongdoer. The word has also been used to characterize the act of an unmarried participator, the other being married. In the United States the definition varies with the local statutes. Unlawful intercourse between two married persons is sometimes called double adultery; between a married and an unmarried person, single adultery.