paramour

paramour [ˈpærəˌmʊər] n.

1.) A person beloved by one of the opposite sex; a ‘love’, a lover, a sweetheart; also of animals.
2.) The lady-love of a knight, for whose love he did battle; hence, the object of chivalrous admiration and attachment.
3.) An illicit or clandestine lover or mistress taking the place, but without the rights, of a husband or wife. Now, the illicit partner of a married man or woman (Oxford English Dictionary 2nd Edition).

Etymology: Middle English adoption of Old French adverb phrase par amur, amour, -s, by or through love. From an early date the phrase was written as one word, and came to be treated (in English) as a noun, both in sense of ‘love’ and ‘beloved, lover’. This may have come partly through a mistaken analysis of the phrase 'to love paramour'.

"While I was your wife, Magnus, you led happy triumphs home:
your fortune changed with your marriage-bed, and that paramour,
Cornelia, condemned by Fate to drag her mighty husbands down
always to disaster, married into a warm tomb."
(Civil War by Lucan, Susan H. Braund (trans.), 1992).

(An Odalisque, Joseph Douglas, 1921)