Pi (letter) different from pi
The mathematical real transcendental (and thus 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.
Homotopy groups  in algebraic topology.
Dimensionless parameters constructed using the Buckingham π theorem of dimensional analysis.
The hadron called the pion (pi meson).
Often inflation rate in macroeconomics.
Sometimes profit in microeconomics.
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.
Policy in reinforcement learning.Polyamory (in the earliest polyamory pride flag design, created by Jim Evans in 1995, pi stands for the first letter of polyamory).