Adultery is,
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