love affair

1. An intimate sexual relationship or episode between lovers.
2. A strong enthusiasm: America's love affair with the automobile.

noun

1. An intimate sexual relationship between two people: affair, amour, love, romance. See love/hatred, sex/asexual.
2. A strong, enthusiastic liking for something: love, passion, romance. See love/hatred.

1.  An intimate sexual relationship, as in They had a torrid love affair many years ago. This expression dates from about 1600, when it referred merely to the experiences connected with being in love. The current sense dates from the second half of the 1800s.
2.  A strong enthusiasm, as in We can't ignore America's love affair with the automobile. [Mid-1900s]

The noun has one meaning:

Meaning #1: a relationship between two lovers
  Synonym: romance