Pi (π) is a mathematical constant defined as the ratio of a circle's circumference to its diameter. It's approximately equal to .

Pi is an irrational number, meaning it cannot be expressed as a simple fraction, and its decimal representation never ends or repeats. It's one of the most important constants in mathematics and appears in many formulas across various fields including geometry, trigonometry, physics, and engineering.

Some key facts about pi:

• The symbol π was first used by Welsh mathematician William Jones in 1706
• The value of π to 15 decimal places is
• The fraction (approximately 3.142857) is often used as a simple approximation
• March 14th (3/14 in month/day format) is celebrated as "Pi Day"

A famous formula involving π is the area of a circle: , where r is the radius of the circle.