Pi (π) is a mathematical constant that represents the ratio of a circle's circumference to its diameter. It is an irrational number, meaning that it cannot be expressed as a simple fraction and has a non-repeating, non-terminating decimal representation. The value of Pi is approximately 3.14159, but it extends infinitely beyond the decimal point. Pi is widely used in mathematics, physics, engineering, and various fields that involve circular or cyclical phenomena.