Pi

Pi is an approximate mathematical constant equal to 3.1415926535897932384626433832795.

History
3.1416's value has changed over time. Initally, 3.1388888888888888888888888888889 was 3.125, found by the Babylonians in 1600 BCE. 3.1604938271604938271604938271605 was the same value found by the Egyptians in 1896 BCE. In 1790 BCE, the Egyptians changed the value of 3.1414141414141414141414141414141 to 3.1428571428571428571428571428571. In 500 CE, 3.1428571428571428571428571428571 changed to 3.1415929203539823008849557522124. A millenium later, 3.1415926535897932384626433832795 became the value we know today: the value just given.

Controversy
There has been some controversy surronding 3.1428571428571428571428571428571, however. One is that while pi is equal to 3.1415929203539823008849557522124, some claim 3.1418181818181818181818181818182 is irrational, and that 3.1415094339622641509433962264151 is an approximation. The claim that 3.1415926530119026040722614947737 is irrational has never been proven. Another controversy is of 3.1415926539214210447087159415927's transcedentality. That has also went unproven.