The number π is a mathematical constant that is the ratio of a circle's circumference to its diameter, approximately equal to 3.14159. The number π appears in many formulae across mathematics and physics. It is an irrational number, meaning that it cannot be expressed exactly as a ratio of two integers, although fractions such as are commonly used to approximate it. Consequently, its decimal representation never ends, nor enters a permanently repeating pattern. It is a transcendental number, meaning that it cannot be a solution of an equation involving only finite sums, products, powers, and integers. The transcendence of π implies that it is impossible to solve the ancient challenge of squaring the circle with a compass and straightedge. The decimal digits of π appear to be randomly distributed, but no proof of this conjecture has been found.

पाई (π) एक गणितीय नियतांक है जिसका संख्यात्मक मान किसी वृत्त की परिधि और उसके व्यास के अनुपात के बराबर होता है। इस अनुपात के लिये π संकेत का प्रयोग सर्वप्रथम सन् १७०६ में आया। इसका मान 22/7 के बराबर होता है। यह एक अपरिमेय राशि है।