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 osmotic pressure in chemistry. Π=MRT

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

The mathematical real transcendental (and thus irrational) constant π ≈ 3.1415926535897932384626433832795028841971…, 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.

Homotopy groups in algebraic topology.

Dimensionless parameters constructed using the Buckingham π theorem of dimensional analysis.

The hadron called the pi meson or pion.

Economic profit in microeconomics.

Inflation rate in macroeconomics.

A type of chemical bond in which the P-orbitals overlap, called a pi bond.

The natural projection on the tangent bundle on a manifold.

The unary operation of projection in relational algebra.

In reinforcement learning π denotes policy.