Pi is the ratio between the circumference and diameter of a circle, where in the above equation C is the circle's circumference and D the diameter.
It seems that all people with a written language, no matter at what point in history, had at least a fundamental knowledge of pi. Degrees of accuracy have changed over time, but little else about pi has.
Pi can also be rendered as 22/7 or 377/120, but is most commonly considered 3.14. Making the number more mysterious is that no one has ever been able to locate a repeating pattern in the decimal sequence. Most decimals repeat a pattern eventually (8.2349574623879879879879, for example), but despite having taken pi out to literally hundreds and thousands of places, no pattern has emerged -- indeed, it is thought to be a truly infinite number.
As with the Golden Ratio, with pi we can thus witness the Macrocosm in the Microcosm; the Infinite within the Finite. It also adds mathematical substance to the role of the circle as a symbol of Infinity.