From The New Unger's Bible Dictionary
FORNICATION (Gk. porneia). Used of illicit sexual intercourse in general (Acts 15:20,29; 21:25; cf. 1 Cor. 5:1; 6:13,18; 7:2; etc). It is distinguished from "adultery" (Gk. moicheia in Matt. 15:19; Mark 7:21). The NIV usually translates porneia as "sexual immorality" and moicheia as "adultery". Jahn (Biblical Archaeology, sec. 158) thus distinguishes between adultery and fornication among nations where polygamy exists: "If a married man has criminal intercourse with a married woman, or with one promised in marriage, or with a widow expecting to be married with a brother-in-law, it is accounted adultery. If he is guilty of such intercourse with a woman who is unmarried it is considered fornication." At the present time adultery is the term used of such an act when the person is married, fornication when unmarried; and fornication may be defined as lewdness of an unmarried person of either sex. Its prohibition rests on the ground that it discourages marriage, leaves the education and care of children insecure, depraved and defiles the mind more than any other vice, and thus makes one unfit for the kingdom of God (1 Cor. 6:9); etc). Our Lord forbids the thoughts that lead to it (cf. Matt. 5:28)
Figurative. The close relationship between Jehovah and Israel is spoken of under the figure of marriage, Israel being the unfaithful wife of the Lord, now rejected but yet to be restored. The church of the NT is a pure virgin espoused to Christ (2 Cor. 11:2) and thus differentiated from the nation of Israel (1 Cor. 10:32). The worship fo idols is naturally mentioned as fornication (Rev. 14:8; 17:2,4; 18:3; 19:2,KJV; NASB, "immorality"; NIV, "adulteries"); as also the defilement of idolatry, as incurred by eating the sacrifices offered to idols (Rev. 2:21,KJV; NASB, and NIV, "immorality).