The word for today is…
paramour (noun) – A lover, especially a lover of a person who is married to someone else.
Source : The Free Dictionary
Etymology : Circa. 1300, noun use of adverbial phrase par amour (circa 1300) “passionately, with strong love or desire,” from Anglo-French and Old French par amour, from accusative of amor “love,” from amare “to love”). Originally a term for Christ (by women) or the Virgin Mary (by men), it came to mean “darling, sweetheart” (mid-14th century) and “mistress, concubine, clandestine lover” (late 14th century).
