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1. The ratio of the circumference of a circle to the diameter of the circle. Often written as the (lower case) Greek letter π.

PI is an irrational number, meaning that if it were written out in full it would never end. For most purposes the value 3.14159265 suffices. The value 22/7 provides a very rough approximation, which is correct to two decimal places only.

For more information see:

2. Primary In.