
Adultery refers to a sexual relationship between two people, at least one of whom is married to someone else. Adultery must be distinguished from fornication, which generally refers to a sexual relationship between two unmarried people (sometimes called 'premarital sex'), and rape, which is a sexual assault and doesn't refer to either person's marital status.
Adultery is condemned across cultures and religions as an act of faithlessness, though the grounds for complaint vary from culture to culture. In some cultures, where the wife is considered her husband's property, adultery is treated as theft and prosecuted accordingly. In other cultures, including most Western cultures, adultery is seen as a failure to keep the obligations of marital fidelity. In all cases, adultery is grounds for divorce.
In divorce proceedings, adultery may be a factor in considering property settlement, custody of children, and alimony. If your spouse committed adultery, you may be entitled to more alimony than if the grounds are simply "irreconcilable differences."
A recent study in USA Today suggests that 10-15% of women and 20-25% of men in the United States have had adulterous relationships, but some studies place the number much higher than that. A 2007 MSNBC survey indicates that about half of all people who responded admitted to committing adultery or cheating in a monogamous relationship.
Whatever the true numbers are, there's no doubt that adultery is a serious threat to the health of any marriage.