The ratio of a circle's circumference to its diameter
In other words: all the way around a circle divided by all the way across it.
The symbol is π
No matter how large or small the circle, its circumference is always π times its diameter.
π = 3.14159265358979323846... (the digits go on forever without repeating)
A rough approximation is 22/7 (=3.1428571...), but that is not accurate.
