I don't know what the symbol is called, but I have figured out a simple and logical (elegant, even) explanation for it.
It is used to convert between degrees and radians, right? In fact, to TURN degrees into radians.
Now, take the symbol for degree - that little raised circle thing.
The symbol for radian is the letter "r".
TURN the "r" upside down and attach it to the degree sign - and voila!
So, basically, it is rebus.