The Greek letter **π** (pi) was derived from the ancient Phoenician letter 𐤐 (pē).

The number π (named so after the first letter of the Greek word περιφέρεια, “circumference”) is a mathematical constant, a transcendental number with approximate value of 3.141592. It is defined as the ratio of the circumference of a circle circumference *C* to its diameter *D* (or to twice the radius *r*):

*C*/

*D*=

*C*/2

*r*

The lower-case letter π is also used as a symbol of:

- in biochemistry: a π-helix, a type of protein secondary structure
- in chemistry: a π bond and π molecular orbital
- in mathematics: the prime-counting function
- in particle physics: a pion (aka pi meson), any of three subatomic particles: π
^{0}, π^{+}, π^{−}