ETYM Latin adulterium. Related to Advoutry.
Extramarital sex that willfully and maliciously interferes with marriage relations; SYN. criminal conversation, fornication.
Voluntary sexual intercourse between a married person and someone other than his or her legal partner.
It is almost universally recognized as grounds for divorce in the US, and is a punishable offense in some states. The legal definition of adultery varies: in some states both parties have to be married (to other people); in others it is adultery if the woman is married but not if only the man is married, and so on.