# Pi

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**π** (/paɪ/) is a mathematical constant. It is defined as the ratio of a circle’s circumference to its diameter, and it also has various equivalent definitions. It appears in many formulas in all areas of mathematics and physics. The earliest known use of the Greek letter **π** to represent the ratio of a circle’s circumference to its diameter was by Welsh mathematician William Jones in 1706.^{} It is approximately equal to 3.14159. It has been represented by the Greek letter “π” since the mid-18th century, and is spelled out as “**pi**“. It is also referred to as **Archimedes’ constant**.