1. Someone who commits fornication or extramarital relations between partners who are not married to each other: As soon as Mary found out that her husband was cheating on her, she called him an adulterer, along with many other terms, and threw him out of the house!
2. Etymology: from Old French fornication, from Late Latin fornicationem (fornicatio; from fornicari "to fornicate"; from Latin fornix, "brothel"; originally "arch, vaulted chamber"; from fornus "arched or domed shape".

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