**Pi** (Π π) is the 16th letter of the Greek alphabet and has a numeric value of 80. In words it is pronounced /p/.

The upper-case letter Π is used as a symbol for:

- In mathematics, the product operation.
- In textual criticism,
*Codex Petropolitanus*, a 9th century, uncial codex of the Gospels, now located in St. Petersburg, Russia.

- In economics, profit.
- In mathematics, the constant pi, which is the ratio of the circumference to the diameter of a circle.
- In number theory, the function π(
*x*) is the number of primes that are less than or equal to*x*. - In particle physics, π
^{0}, π^{+}and π^{-}are three forms of pi meson.