π (pi) is the ratio of a circle’s circumference to its diameter. For any circle, this ratio is constant:

π ≈ 3.14159265359…

It’s an irrational number (its decimal expansion never ends or repeats) and also transcendental, meaning it is not the solution to any polynomial equation with integer coefficients.

π appears throughout mathematics, from geometry and trigonometry to complex analysis and probability. It also shows up in physics, particularly in wave behaviour and signal processing.

Further reading