Pi is a mathematical constant that represents the ratio of a circle's circumference to its diameter. It is an irrational number, meaning it cannot be expressed as a simple fraction, and its decimal representation goes on forever without repeating. The value of pi is approximately 3.14, but it has been calculated to trillions of digits. Pi is widely used in various fields, including mathematics, engineering, and physics. It plays a crucial role in calculations involving circles, such as finding the area or circumference. The symbol for pi (π) was first used in the early 18th century and has since become a fundamental concept in geometry.