Involving a romantic or sexual relationship with someone other than your spouse.
From Latin 'extra' (outside) + 'maritalis' (relating to marriage). The prefix 'extra-' meaning 'beyond' was combined with the Medieval Latin term for marriage in the 16th century to describe affairs outside wedlock.
Interestingly, the phrase 'extramarital affair' is almost always hyphenated or one word, but you'll rarely hear people say 'extramarital friendship'—the word has become so specifically tied to infidelity that using it any other way sounds awkward, showing how powerfully context shapes word meaning.
Complete word intelligence in one call. Free tier — 50 lookups/day.