Wikipedia: The mathematical constant π is an irrational real number, approximately equal to 3.14159, which is the ratio of a circle's circumference to its diameter in Euclidean geometry.
Pi is special because , no matter how many decimal places you take it to, it never repeats a number or a sequence of numbers.
The numerical value of π truncated to 500 decimal places is:

3.1415926535897932384626433832 795028841971693993751058209749 44592307816406286
208998628034825342117067982148 086513282306647093844609550582 231725359408128481 117450284102701938521105559644 622948954930381964428810975665 933446128475648233 786783165271201909145648566923 460348610454326648213393607260 249141273724587006 606315588174881520920962829254 091715364367892590360011330530 548820466521384146 951941511609433057270365759591 953092186117381932611793105118 548074462379962749 567351885752724891227938183011 94912
The value of π has been known in some form since antiquity. As early as the 19th century BC, Babylonian mathematicians were using π = 25⁄8, which is within 0.5% of the true value.

The Egyptian scribe Ahmes wrote the oldest known text to give an approximate value for π, citing a Middle Kingdom papyrus, corresponding to a value of 256 divided by 81 or 3.160.

It is sometimes claimed that the Bible states that π = 3, based on a passage in 1 Kings 7:23 giving measurements for a round basin as having a 10 cubit diameter and a 30 cubit circumference. Rabbi Nehemiah explained this by the diameter being measured from outside rim to outside rim while the circumference was the inner brim; but it may suffice that the measurements are given in round numbers.


Principle of Archimedes' method to approximate πArchimedes of Syracuse discovered, by considering the perimeters of 96-sided polygons inscribing a circle and inscribed by it, that π is between 223⁄71 and 22⁄7. The average of these two values is roughly 3.1419.

The Chinese mathematician Liu Hui computed π to 3.141014 in AD 263 and suggested that 3.14 was a good approximation

Pi is the relationship between the area of a circle and the square of the radius of that circle.
Aprox value is 3.14

Pi is the ratio of the circumference of a circle to its diameter. To put it another way, it's the length of the circumference divided by the length of the diameter.

Pi always works out to be the same value, no matter what size the circle is. So Pi is a Big Deal because it lets you easily measure the length of the curved line that makes up a circle.

Pi = C/D, so if you know the diameter and Pi, you can calculate the circumference.

IF you do the math, and divide the circumference by the diameter, you'll get 3.1415926535897932384626433832 7950288419716939937510.....
-- where the dots represent the fact that no matter how long you keep dividing, it never comes out even and the pattern of numbers doesn't repeat. It has reportedly been calculated out to 206 billion digits!!

There are charts that describe the history of Pi, which goes back to the Babylonians (2000 BC), and how accurately (or inaccurately) Pi has been calculated through the years. I'm sorry I can't make it appear in neat columns here, but you can see it neatly if you go to http://www-gap.dcs.st-and.ac.uk/.../Pi_chronology.html

Even though the rough value of Pi was known long before his time, the first theoretical calculation seems to have been carried out by Archimedes of Syracuse (287-212 BC).
--- http://www-gap.dcs.st-and.ac.uk/.../...ugh_the_ages.html

I have altered the answer in response to comments from readers. Sorry for any mistakes.

Consider any circle of any radius.
The circumference of this circle always is little more than 3 times its diameter.
And this "little more" is always one-seventh of its diameter.
So the circumference is always 3+(1/7) of its diameter, which you can write as (21/7)+(1/7)=(22/7).
This value, 22/7 is called as "pi".

Pi or π is one of the most important mathematical constants, approximately equal to 3.14159. It represents the ratio of any circle's circumference to its diameter in Euclidean geometry, which is the same as the ratio of a circle's area to the square of its radius. Many other formulas from mathematics, science, and engineering include π. It is an irrational number, which means that its decimal expansion never ends or repeats.

For more information on Pi visit http://en.wikipedia.org/wiki/Pi

It doesn't have almost infinite digits. It was discovered that it has about 12 trillion decimal places, which is no where near infinity. :-)

opps I learned that in 7th grade im sorry!

Pi is 3.14159265358979323846 etc. thats on top of my head! Pi is 200bc years old and you know what im only 7th grade and i know that!Pi keeps going on forever it never stops! PI day is officially march 14th! PI IS THE NUMBER OF TIMES A CIRCLE'S DIAMETER WILL FIT AROUND ITS CIRCUMFERENCE!PI IS THE 16TH LETTER OF THE
GREEK ALPHABET.In 1706 William Jones first gave the Greek letter "π" its current mathematical definition.All the digits of Pi can never be fully known.Three point one four one five nine two
It’s been around forever - its not new
It appears everywhere in here and in there
Its irrational I know but its true!The 3,000,000,000th decimal digit of PI is an 8. There thats all I know about Pi ask more questions and ill answer them!

It slays the natives if you can tell them that pi equals 3.1415926--which is easy to remember from the number of letters in the phrase "Yes, I want a drink; alcoholic of course."

But what is the Platonic marriage number, where the product of pi digits 3141592654 = 129,600?