My understanding was that it was a form of "extramarital" romance for knights and nobles in Europe during the Middle Ages. Was it ever an "institution" anywhere, and what (if any) impact did it have on society?
Here's the version I've been given. (I posted the question to solicit other answers as a "cross-check.") My answer differs from the others insofar as it (supposedly) occurred in the LATE, rather than early, Middle Ages.
Apparently, it was a life cycle management tool.
The cycle would begin when an "established" knight of around 35 married a young noblewoman aged about 20. And they would usually begin a family.
About ten years later, when the husband was about 45 and the wife about 30, she would begin an (initially) platonic relationship with an "apprentice" knight (or "knight errant" in the prevailing terminology), some 10-15 years younger, in his late teens.
She would seal the relationship by giving him a personal token such as a ring or handkerchief, which he would wear. This was known as "wearing a lady's favor." These liaisons were quite open, insofar as "everyone" knew which knight was wearing which lady's favor. And as will be seen, this served as a form of "engagement."
The young knight errant would then go off to war, (hopefully) returning some eight or ten years later. By that time, the husband would (probably) be dead (medieval life expectancy was something like early 50s for the nobles, much less for peasants), the lady would be about 40, and the young knight would be in his mid-to-late 20s. At this point, the relationship would be consummated.
They might sometimes marry, mostly not, especially if the lady had children. Eight or ten years after this, the lady would be dead (or close to), while the knight would be in his mid to late 30s, and begin the cycle again by taking a 20-ish wife.
Did such couples "cheat" (by having sex while the husband was still alive)? Of course some did. But most did not. Ladies usually held off until about age 40 1) so that their husbands and lovers would die naturally and not kill each other in duels, and 2) until after they became post-menopausal in order not to have two sets of children.
The need arose because of the "conventional" 15 year age difference between an "established" knight and a barely post-pubescent bride. Courtly love would provide "second husbands" for all the widows involved, and "Mrs. Robinsons" to help transition graduated knights into married life.
I read somewhere that this was an impetus for the Crusades. Can't provide real sources, but it might make sense intuitively, insofar as there are a bunch of young knights errants seeking to prove themselves to their ladies (and hoping for a "reward" when they return).
You can get a fairly comprehensive answer here. Honestly, that seems like it is a good source and it certainly agrees with the Geis family. I'm only including the comment below because I wrote it before finding that link (and I really like the story I relate in it).
The book, "Life in a Medieval City" (by the Geis family) actually has an entire chapter on the subject. Several important points can be summed:
A story related in the same book is was something akin to:
As an aside, an example of this love can be found in the story, "Sir Gawain and the Green Knight." (I recommend Tolkien's version) Sir Gawain fights valiantly and is victorious, except that he has failed to be forthcoming about his interactions with his hosts' wife. Read the whole tale if you're interested in learning how it works out.
As far as I know, it was a platonic love which could go as far as non penetrative sex. Certainly having children was a big no no -- that would be adultery. Although, I am unaware of any formal definitions or codifications. In an age and class where marriage was for political gain, it was seen as a way to actually love someone one was not married to. I believe, but have no evidence, to say it was mostly a romantic idea found in novels more than a real social accepted norm.
Henry of Navarra and the Queen Margot could have been said to partake of "amour courtois" outside of their own marriage to each other. Although, I do not believe it was ever refereed to as such.
One of the impacts would be in the Arthurian legends where if Arthur was not so jealous, he would have allowed an amour courtois between Lancelot and Guinevere. Instead, he precipitated the fall of the round table. Jealousy is bad.