ΠπPi is a letter in the Greek language alphabet of Earth, originating in the early period of Human history.
Pi is also a mathematical constant that was the ratio of any circle's circumference to its diameter, which is approximately 3.1416.
In 2267, when the entity Redjac took control of the computer of the USS Enterprise, Spock forced it out with a Class-A compulsory directive to compute pi to the last digit – a task that it could never complete, because, as Spock explained, "the value of pi is a transcendental figure without resolution". (TOS: "Wolf in the Fold")
Data, who was sharing amusing anecdotes with Lwaxana Troi and Jean-Luc Picard in late 2365, explained how he once used the square root of pi and multiplied it by 93 to accurately calculate the distance between the Omicron system and the Crab Nebula. (TNG: "Manhunt")
When Odo and Quark were climbing a mountain after their crash on a class L planet, Odo asked how much higher they needed to climb, and Quark responded by saying "carry the seven, take the square root, times pi. I'd say that high." (DS9: "The Ascent")