Pi (π)
The ratio of a circle's circumference to its diameter — an irrational number that begins 3.14159… and never terminates or repeats.
Origin
Pi has been studied for nearly 4,000 years. Babylonians approximated it as 3.125; the Egyptians as 3.16; Archimedes (c. 250 BCE) bracketed it between 3.1408 and 3.1428 using polygons inscribed in circles. The Greek letter π was first used for it by William Jones in 1706 and popularized by Euler. It was proved irrational in 1761 and transcendental (not the root of any polynomial) in 1882. Modern supercomputers have calculated it to over 100 trillion digits.
Modern usage
March 14 (3/14) is Pi Day — celebrated with pies, recitation contests, and obligatory math-teacher jokes. Pi shows up far beyond circles: in probability, in the equation for the period of a pendulum, in Heisenberg's uncertainty principle. Memorizing many digits of pi is a recurring nerd benchmark.
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