Someone who commits adultery or fornication (sexual intercourse between partners who are not married to each other).

The word fornication had a less than socially respectable beginning suitable to what has long been the low moral status of the act to which it refers. From Old French fornication, from Late Latin fornicationem (fornicatio), from fornicari "fornicate", from Latin fornix, "brothel"; originally "arch, vaulted chamber"; from fornus "oven of arched or domed shape".

Roman prostitutes commonly solicited customers from under the arches of certain buildings. So, fornication means, "voluntary sex between an unmarried man and an unmarried woman"; extended in the Bible as adultery.