paramour
/ˈpærəmɔːr/Definitions
1. noun
a lover or sweetheart, especially one that is secret or illicit
“The scandal centered around her alleged affair with a married man, who was revealed to be her paramour.”
2. noun
a person with whom one has a close and often romantic relationship, especially a romantic or sexual relationship outside marriage
“The paramour of the young nobleman was a commoner from a lower social class.”