Pi (upper case Π, lower case π) is the 16th letter of the Greek alphabet. In the system of Greek numerals it has a value of 80.

In Greek, the letter is pronounced /piː/; in English, though, it is pronounced /paɪː/ (as the word pie). In words, it is pronounced /p/. In Modern Greek, the sequence of letters μπ represents the /b/ sound, as in boy (the second letter of the Greek alphabet is now pronounced /v/ as in very).

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

• The product operator in mathematics (similar to a series).
• In textual criticism, Codex Petropolitanus, a 9th century, uncial codex of the Gospels, now located in St. Petersburg, Russia.

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

• The mathematical constant π ≈ 3.14159, the ratio of a circle's circumference to its diameter.
• The prime counting function in mathematics.
• Dimensionless parameters constructed using the Buckingham Pi theorem of dimensional analysis.
• The elementary particle called the pi meson or pion.
• Profit in microeconomics.
• Inflation rate in macroeconomics.