I pulled up the Websters Dictionary on my Bible Study Program E Sword
Here is the difference
Adultery
ADUL'TERY, n. [L. adulterium. See Adulterate.]
1. Violation of the marriage bed; a crime, or a civil injury, which introduces, or may introduce, into a family, a spurious offspring.
By the laws of Connecticut, the sexual intercourse of any man, with a married woman, is the crime of adultery in both: such intercourse of a married man, with an unmarried woman, is fornication in both, and adultery of the man, within the meaning of the law respecting divorce; but not a felonious adultery in either, or the crime of adultery at common law, or by statute. This latter offense is, in England, proceeded with only in the ecclesiastical courts.
In common usage, adultery means the unfaithfulness of any married person to the marriage bed. In England, Parliament grant absolute divorces for infidelity to the marriage bed in either party; and the spiritual courts divorce a mensa et thoro.
2. In a scriptural sense, all manner of lewdness or unchastity, as in the seventh commandment.
3. In scripture, idolatry, or apostasy from the true God. Jer 3.
4. In old laws, the fine and penalty imposed for the offense of adultery.
5. In ecclesiastical affairs, the intrusion of a person into a bishopric, during the life of the bishop.
6. Among ancient naturalists, the grafting of trees was called adultery, being considered as an unnatural union.
Fornication
FORNICA'TION, n. [L. fornicatio.]
1. The incontinence or lewdness of unmarried persons, male or female; also, the criminal conversation of a married man with an unmarried woman.
2. Adultery. Mat 5.
3. Incest. 1 Cor 5.
4. Idolatry; a forsaking of the true God, and worshipping of idols. 2 Chr 21. Rev 19.