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Pi (π) is a constant describing the ratio between a circle's circumference and its diameter. It is an irrational number usually summarized to two decimal places as 3.14.
The Greek letter for P was chosen by the 18th century Welsh mathematics teacher William Jones, most likely to stand for 'periphery'.
Symbolically, the term pi was chosen to stand for something more than a mere number. Prior to its use in the early 18th century, the quantity was represented by terms and fractions, none of which adequately reflected an incomprehensible, infinitely long sequence of non-repeating decimals.
Jones might have suspected that "'the exact proportion between the diameter and the circumference can never be expressed in numbers", but it wasn't until the 1760s that the Swiss polymath Johann Lambert came up with a proof of its irrationality.