>>/41957/
do you have a reference? I don't know what you mean. Granted... I was never tasked with trying to search for more answers myself, I was simply told what it really was.

also, if you want to describe it, picture a big cicle. Now, inside of that circle, picture a square, where the corners meet tangentially with the rim of the circle. 
Now, view the area within the circle but not the area within the square.
Behold, now you see.