Adultery is the voluntary sexual intercourse between a married person and another person who is not his or her spouse, though in many places it is only considered adultery when a married woman has sexual relations with someone who is not her husband and in others it is only considered adultery when a married woman has sexual relations with someone without the permission of her husband.
In most cases, in Western countries, only the married party is said to have committed adultery, and if both parties are married (but not to each other) then they both commit separate acts of adultery. In other countries, both parties to the adultery are considered guilty, while in others again only the woman is able to commit adultery and to be considered guilty