Pi (uppercase Π, lower case π) is the sixteenth letter of the Greek alphabet. In the system of Greek numerals it has a value of 80. In Modern Greek, the name of the letter is pronounced /pi/; in modern English, it is pronounced /paɪ/, in particular when referring to the mathematical constant (see below). In words, it is pronounced /p/. In Modern Greek, the sequence of letters μπ represents the /mb/ 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, indicated with capital pi notation ∏ (in analogy to the use of the capital Sigma Σ as summation symbol).
- In textual criticism, Codex Petropolitanus, a 9th century, uncial codex of the Gospels, now located in St. Petersburg, Russia.
- In legal shorthand, it represents a plaintiff.
The lower-case letter π is used as a symbol for:
- The mathematical irrational constant π ≈ 3.14159..., the ratio of a circle's circumference to its diameter in Euclidean geometry. The letter "π" is the first letter of the Greek words "περιφέρεια" 'periphery' and "περίμετρος" 'perimeter', i.e. the circumference.
- The prime-counting function in mathematics.
- Dimensionless parameters constructed using the Buckingham π theorem of dimensional analysis.
- The osmotic pressure in chemistry. π=MRT
- The elementary particle called the pi meson or pion.
- Profit in microeconomics.
- Inflation rate in macroeconomics.
- A type of chemical bond in which the P-orbitals overlap
- The natural projection on the tangent bundle on a manifold.
"Variant pi" or "pomega" (<math>\varpi\,\!</math> or ϖ) is a glyph variant of lower case pi sometimes used in technical contexts as though it were a lower-case omega with a macron, though historically it is simply a cursive form of pi. It is used as a symbol for
- The angular frequency of a wave in fluid dynamics (angular frequency is usually represented by <math>\omega</math> but this may be confused with vorticity in a fluid dynamics context).
- The longitude of pericenter in celestial mechanics. [1]
- Comoving distance in cosmology. [2]
In HTML, the capital letter Π can be produced using the codes Π or Π, and the lower-case by using π or π within the source code.
