I was born on pi day ;-) Seriously, the linear circumference of a circle divided by that same circle's diameter, will equal pi (3.14..). But pi is an infinite and irrational number (can't be produced using one integer divided by an other integer) and "transcendent". It's among the most important constants in mathematics - even if it doesn't end (we have mathematical proofs of this claim) nor resolve to a repeating decimal. Scientists often require pi's resolution to be as accurate as 40 decimal places. Irrational numbers often drive high school students up the wall.
Pi is defined as most have interpreted it in the other responses. Pi is the ration of the circumference of a circle to its diameter. This number is a constant. As it turns out this ratio, although this value, which is the same for all circles, the value is such that the decimal places of the number do not form a repeating pattern (including "ending" which is a repeating pattern of zeros at some place. Thus, the phrase "never ending" is applied to Pi. Pi *is irrational. Irrational means the number cannot be fully expressed as the ratio of two integers, also stated by others. Pi is *transcendental* (as opposed to transcendent as reported otherwise). "Transcendental" means there is no polynomial equation with rational coefficients for which the number is a solution. Pi is a "real number" as in expressible as a decimal value, albeit a never ending, never repeating decimal value. So, traditionally the "real numbers contain the rational and irrational numbers, and transcendental numbers are a more specialized subset of the irrational numbers. Square root of 2 is irrational, but not transcendental, for example.
a circle ends in its self so there for that couldnt be the answer, the reason pi is so long is because it gives math matics a way to interact with numbers all day long, couldnt someone just make an app that does the math of pi so we can process this information and get it out of the way!
pi does end because it gets smaller and smaller. the reason why people can't find the end of it is because our units of measurement don't touch it. like if your using feet to measure something that is 9 inches. our units of measurement just happen to be too big or too small. if it was adjusted we would see pi