paramour
/ˈpærəˌmʊə(r)/noun
- archaic or derogatory a lover, especially the illicit partner of a married person〈古或贬〉情人（尤指情妇、情夫）。
词源
Middle English: from Old French par amour 'by love'; in English the phrase was written from an early date as one word and came to be treated as a noun.