Pi is the name for the Greek letter π, which corresponds to the English letter p.
π is also the symbol used in mathematics for the ratio between the diameter of a circle and its circumference, and which appears in many other places.
π is an irrational number; this means that it cannot be expressed as a fraction, and (therefore) cannot be expressed exactly as a decimal no matter how many decimal places it is carried out to.
The value of π is approximately 3.14159. This value is precise enough for almost all ordinary purposes; it can, for example, be used to calculate the circumference of the Earth with an error of only 350 feet.
For rough purposes, the fraction 22/7 (= 3.14285...) is sometimes used.
To some extent, the progress of mathematics—or at least of computation—can be gauged by the progress in the number of digits to which π has been calculated. In 1873 Abraham Shanks spent twenty years calculating π to 707 places, but unfortunately made a mistake in his calculation and only 527 of them were correct. When electronic computers were developed, π was soon calculated to tens of thousands, millions, and billions of places. As of 2002, the record is held by Yasumasa Kanada of Tokyo University at 1,241,100,000,000 digits. That result was never printed out: can you figure out why not?
Memorizing π is a challenge that appeals to some people. Mnemonics have been devised. Counting the letters in the phrase "Now I wish I had a drink—alcoholic, of course" give π to nine places, which is more than enough for all ordinary purposes.
In the Bible, 1 Kings 7:23 contains the famous passage "And he made a molten sea, ten cubits from the one brim to the other: it was round all about, and his height was five cubits: and a line of thirty cubits did compass it round about." If it assumed that "round" means "circular," it seems to imply that the value of π is exactly 3.