Le Parkour (also called Parkour, PK, l’art du déplacement, free-running) is a physical discipline of French origin. It is an art form of human movement, focusing on uninterrupted, efficient forward motion over, under, around and through obstacles (both man-made and natural) in one’s environment. Such movement may come in the form of running, jumping, climbing and other more complex techniques.
It doesn’t interest me as an activity, but the word is notable in that it’s a borrowing from French in nonstandard spelling, something of a rarity. As a result, when you look at it in English it’s not clear how to pronounce it; if it had the standard spelling parcours that wouldn’t be a problem. (Frankly, I find this kind of respelling with k for c pretty ugly, but I guess that’s the point, or part of it. Epater le fuddy-duddy, you know.)
Incidentally, the same MeFi thread introduced me to the word thixotropic; see my first comment therein for more.