Unfaithfulness of any married person to the marriage bed
(a married person having sex with someone not his or her spouse).
In a scriptural sense,
all manner of lewdness or unchastity,
as in the Seventh Commandment.
(See Webster's 1828)

Now the works of the flesh are manifest,
which are these;
Adultery, fornication, uncleanness, lasciviousness,
Idolatry, witchcraft, hatred, variance, emulations,
wrath, strife, seditions, heresies,
Envyings, murders, drunkenness, revellings, and such like: of the which I tell you before, as I have also told you in time past, that they which do
such things shall not inherit
the kingdom of God.

Galatians 5:19-21