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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 'pee'; in English, though, it is pronounced '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.
• The elementary particle called the pi meson or pion.
• ProfitProfit is what is gained, after costs are accounted for. In accounting, this is usually measured in monetary terms. In economics, profit is most often measured differently, since costs are opportunity costs. Profit is income received by buying low and sel in microeconomics.
• InflationFor alternative meanings see inflation (disambiguation). In economics, inflation is a fall in the market value or purchasing power of money. This is equivalent to a rise in the general level of prices. Inflation is the opposite of deflation. Zero or very rate in macroeconomics.