Adultery is a term which refers to extramarital sex or sex outside of a monogamous marriage. To have sexual relations with someone other than one's own husband or wife is to commit adultery.
More About Adultery
The specific acts which constitute adultery and the legal and moral implications of those acts vary between cultures and religions. Adultery can be grounds for divorce or may void a pre-nuptual agreement. Generally, the consequences of adultery are negative.
