Pi (π) is the ratio of the circumference of a circle to the diameter of the circle. No matter the size of the circle, pi is always the same.
Pi is important to geometry and trigonometry, of course, because of its relation to circles, ellipses, and spheres. But it is also found in other fields of study, such as cosmology, number theory, statistics, fractals, thermodynamics, mechanics, and electromagnetism. Pi seems to be everywhere. That is why it is one of the most widely known mathematical constants.
Pi is sometimes approximated as 3.14 or as the fraction 22/7, but pi is actually an irrational number, meaning it cannot be expressed as a fraction and its decimal places continue infinitely with no pattern. If you wrote it out in full (which is impossible), its decimal places would continue forever.
Pi is also believed to be a normal number, which means simply that no digit, or combination of digits, occurs more frequently than any other. Being infinitely long and completely random, means that any string of numbers can be found somewhere in the digits of pi. Pi contains every phone number in the world, and if you converted numbers to letters, you’d find every book that has ever been written. You can search for your own phone number (or any other number) in the first 200 millions digits of pi at The Pi-Search Page. (If you don’t find a number it doesn’t mean it isn’t there, just that it is not in the first 200 million digits. Remember that pi is infinitely long.)