par·a·mour 
(părə-mr′)
(părə-mr′)n.
A lover, especially a lover of a person who is married to someone else.
[Middle English, from par amour, by way of love, passionately, from Anglo-Norman : par, by (from Latin per; see per1 in the Appendix of Indo-European roots) + amour, love (from Latin amor, from amāre, to love).]