When one spouse, goes outside of their marriage for sexual relations, with a third party.
Noted within many Ancient civilizations, though under different words, most notably 'adultery' as contained within biblical references.
Mainly where one spouse, cheats on the other.
The 3rd party may or may not also be involved in a supposed monogamous relationship. (marriage).
A more tame word, to denote cheating, adultery.
Practice (Associated Acts):
Many euphemisms exist for this term, such as:
Bit on the side, bit of skirt, fooling around, fucking around, hitting the triangle, moonlighting, messing around, double dealing, extracurricular activity, playing the field, offshore drilling, monkey business, as well as many more terms, to basically describe cheating.
In some countries, and in most religions, the act of having an extramarital affair, is believed to be a sin, a crime.
Punishment, even today in some Islamic countries, is stoning.