Number, approximately 3.14
Pi A
The number π is a mathematical constant, approximately equal to 3.14159, that is the ratio of a circle's circumference to its diameter.
It appears in many formulae across mathematics and physics, and some of these formulae are commonly used for defining π, to avoid relying on the definition of the length of a curve.
Text adapted from Wikipedia, CC BY-SA 4.0. Photograph: DXR (see Commons), Wikimedia Commons.