extramarital intercourse; adultery
A formal, clinical term for sexual relations outside marriage. Often used in legal, medical, or sociological contexts. In everyday speech, 不倫 (ふりん) or 浮気 (うわき) are more common.
Extramarital intercourse is legally punishable in some countries.
This survey revealed the actual state of extramarital intercourse.
Common term for an extramarital affair, often implying an ongoing romantic relationship. More colloquial than 婚外性交.
Casual term for cheating or infidelity, not necessarily involving full intercourse. Less formal than 婚外性交.
Compound of 婚外 (こんがい, 'outside marriage') and 性交 (せいこう, 'sexual intercourse'). A modern, formal coinage.