PI

PI may refer to:

Arts and media

Partido Independiente or Independent Party, a political party in Uruguay
Partido Intransigente or Intransigent Party, an Argentine political party
Partit per la Independència, a political party in Catalonia (Spain) of the 1990s

Perimeter Institute for Theoretical Physics, a research centre in Ontario, Canada
Privacy International, a UK-based charity that supports the right to privacy across the world
Defunct Piedmont Airlines (1948–89) (IATA airline code PI)
Philadelphia International, record label founded in 1971

Parental investment, in evolutionary biology
Paternity Index, a value used to calculate probability of paternity
Pistillata, a gene that influences the development of flowers in the ABC model of flower development
Phosphatidylinositol, a class of lipids
Ponderal index, a measure of leanness of a person (similar to body mass index)
Propidium iodide, a chemical used as a DNA stain
Primary immunodeficiency
Protease inhibitor (pharmacology), class of drugs used to treat or prevent infection by viruses, including HIV and Hepatitis C
Protease inhibitor (biology), molecules that inhibit the function of proteases
Pulsatility index, a ratio of blood flow rates

Isoelectric point (pI), the pH at which a particular molecule or surface carries no net electrical charge.
Power integrity, in digital electronics
PI controller, a concept in automation and control engineering
Plasticity index, a measure of the plasticity of a soil
Polyimide, a polymer of imide monomers
Preußische Instruktionen, a cataloging system for libraries
Principal investigator, the lead scientist or engineer for a particular project

Pass interference, a foul in American and Canadian gridiron football
People's Initiative, one of the modes in which the constitution of the Philippines could be amended
Philippine Islands (P. I.), the commonly used name of the Philippines during the US colonial period
Pirot, a city located in south-eastern Serbia (license plate code PI)
Political incorrectness or politically incorrect, commonly abbreviated to PI or PIC
Pro forma invoice, in business
Private investigator (P.I.), a person who can be hired to undertake investigations
Profitability index, the ratio of payoff to investment of a proposed project
Personal injury