WordNet (r) 3.0 (2006):

love affair
    n 1: a relationship between two lovers [syn: love affair,
         romance]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "love affair":
   adulterous affair, adultery, affair, amour, cuckoldry,
   entanglement, eternal triangle, flirtation, forbidden love,
   hanky-panky, illicit love, infidelity, intrigue, liaison, love,
   romance, romantic tie, triangle, unfaithfulness