The English word “fornication” generally refers just to voluntary sexual intercourse between two unmarried persons, though it sometimes include adultery. In the New Testament the word often translated “fornication” is the Greek word porneia, which has a much broader semantic field.
The word porneia derives from the word porne (“harlot”). It was used rarely in classical Greek, and referred fornication (not necessarily with a harlot). In the Greek translation of the Old Testament it is used much more widely, describing such things as harlotry, fornication, adultery and cultic prostitution (which was common in many pagan religions). In Jewish literature between the Testaments we find porneia used to describe any kind of unlawful sexual act, including incest, sodomy and unlawful marriage. The New Testament uses the word in the same way, to describe every kind of unlawful or unnatural sexual act.
The word is also used a figurative sense in both the Old and New Testaments. For example, in Revelation 19:2 it is described how the great city Babylon (often interpreted as Rome) corrupted the world through “her immorality” (porneia). This does not mean literal sexual acts but being seduced away from God. Throughout the Old Testament God describes his relationship to Israel as a man to his wife. In the New Testament the relationship between Christ and the church is described in a similiar way. So unfaithfulness (through idolatry) was described metaphorically as adultery and/or fornication.