The colloquial term for the back of the knee, or if you'd like to be technical about it, popliteal fossa. It's the area where your knee bends. Since it's similar to where the arm bends at the shoulder, the arm pit, this area is also called the "knee pit".The above photo of Kanu in cosplay is by Ren of Zark&Ren on Cosplay.com. Look through more of their cosplay at their site!
Please be wary that there may be some picture that are risque and NSFW...
