love affair 
noun
1A romantic or sexual relationship between two people who are not married to each other: she had an unhappy love affair with a painter...
- How could he possibly tell his parents that he was involved in a love affair with a married woman?
- Her relationship with William was depicted as a love affair, involving many partings and returns from war.
- After an unhappy love affair, Brennan moved to New York where her real story starts.
Synonyms
relationship, affair, romance, liaison, affair of the heart, intrigue, fling, amour, involvement, amorous entanglement, romantic entanglement, entanglement;
flirtation, dalliance;
French affaire de/du cœur
British informal carry-on
1.1An intense enthusiasm or liking for something: he had a lifelong love affair with the cinema...
- Nationalism is about an intense love affair with the homeland.
- I eventually bought all their albums and began a life-long love affair with Canadian rock.
- It was here that his lifelong love affair with European culture began.
Synonyms
enthusiasm, mania, devotion, passion, liking, appreciation