term:

Adultery

adultery n

: voluntary sexual activity (as sexual intercourse) between a married man and someone other than his wife or between a married woman and someone other than her husband
;also
: the crime of adultery compare fornication
adul·ter·er [ə-dəl-tə-rər]
n

adul·ter·ess [-tə-rəs]
n

Source: Merriam-Webster's Dictionary of Law ©1996. Merriam-Webster, Incorporated. Published under license with Merriam-Webster, Incorporated.