In the broadest sense, an affair is something—an at least somewhat special something—that’s going on. That’s a fairly broad definition that can cover a number of different types of affairs. For example, it could be a business affair, a professional affair, a social, the sort of affair that probably popped into your mind when you read the word affair, or probably a few sorts of affairs that that sort of affair has pushed out of my mind.
In short, it is an occasion or matter, but only an occasional occasion or matter. If it’s an event that happens regularly or something that you deal with repeatedly as a matter of course, say every night or every week, it would generally not be referred to as an affair. An exception is a sexual affair, which, in addition to possibly occurring only sporadically, might happen every night, every lunch time or at any other time or frequency.
Under the word affair, the dictionary I’m using also lists a French expression, affaire de cœur. I’m always so pleased when I’m presented with a French word that I know. Cœur means heart. I didn’t even have to look it up.
I also knew that de is the French word for “of.” Score another point for me.
I’m not going to claim any points for affaire because the French word affaire means pretty much what the English word affair means. So that one doesn’t count.
Thus, an affaire de cœur, or I should say une affaire de cœur translates literally to an affair of the heart. However, that doesn’t do it justice. An affaire de cœur is a brief, passionate romance between two unmarried people. Some people have all of the fun.