Definition of Fornication
Sexual intercourse other than between married persons. State v. Cook, App., 139 Ariz. 406, 678 P.2d 987, 989. Further, if one of the persons be married and the other not, it is fornication on the part of the latter, though adultery for the former. In some jurisdictions, however, by statute, it is adultery on the part of both persons if the woman is married, whether the man is married or not. This offense, which is variously defined by state statutes, is very seldom enforced. See also Adultery; Illicit cohabitation.
That's the definition of Fornication in Black's Law Dictionary 6th Edition. Courtesy of Cekhukum.com.