Pi is so called because the Greek word for "measure about" starts with the letter P Bashny.Net

Pi expresses the ratio of the circumference to the diameter and approximately 3, 14. It was first designated by the Greek letter π Englishman William Jones at work "Review of the achievements of mathematics", printed in 1706. He was guided by the fact that it begins with the word περιμετρέο - «measure around." Widespread received this designation because of the great mathematician Leonhard Euler (1707-1783), who often used it. How and when was discovered the sheer number is unknown. The fact that the ratio of the circumference to its diameter - the number of permanent, known since time immemorial. The Babylonians in the III millennium BC. e. already knew that pi is equal to or slightly greater than three. Calculate the value of the number of up to three characters it was only in the III century BC. e. Archimedes. And in the XVIII century, Johann Lambert proved that pi can not be expressed as a ratio of two integers, ie in the form of a finite or recurring decimal. By the time the Lambert pi has been calculated with an accuracy of more than a hundred characters. And in the summer of last year it reached 5 trillion digits accuracy.

via factroom.ru