Adultery, fornication and incest designate forms of illicit sexual intercourse which are clearly distinguished in legal use, both civil and ecclesiastical.
Adultery implies unfaithfulness to one’s spouse, and therefore can be applied only to sexual intercourse on the part of a married man with a woman other than his wife, or of a married woman with a man other than her husband.
Fornication designates sexual intercourse on the part of an unmarried person; when occurring between a married and an unmarried person, the former is involved in adultery and the latter in fornication.
Incest designates sexual intercourse between persons so closely related that their marriage is prohibited by church or state and usually by both.