Extramarital (adjective)

Outside the bounds of marriage; not within the confines of matrimony.

Origin:

Late 19th century: from extra- + marital.

Examples:

1. The extramarital affair was a source of much scandal and public outrage.
2. The extramarital relationship was discovered and led to the breakup of the marriage.
3. The extramarital partner was seen as a homewrecker by many people.
4. The extramarital dalliance was kept secret for as long as possible.
5. The extramarital fling was brief and not taken seriously by either party.