π

π =
3.14159265358979323846264338327950288419716939937510582097494459230781640628620899862803482534211706798214808651328230664709384460955058223172535940812848111745028410270193852110555964462294895493038196442881097566593344612847564823378678316527120190914564856692346034861045432664821339360726024914127372458700660631558817488152092096282925409171536436789259036001133053054882046652138414695194151160943305727036575959195309218611738193261179310511854807446237996274956735188575272489122793818301194912983367336244065664308...

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Pi is the ratio of any Euclidean circle's circumference to its diameter. It's also a really f-ing long number that some people get off on memorizing as many of its digits as possible.

π is the Greek letter that corresponds to our 'p' and has been the symbol for the above number since 1706, though the notation wasn't popularized until Leonard Euler used it in 1737. Note that Π is also pi, but it means something totally different in mathematical notation.