Psi

Psi or PSI or Ψ may refer to:

Alphabetic letters

Psi (Greek) (Ψ, ψ), 23rd letter of the Greek alphabet
Psi (Cyrillic) (Ѱ, ѱ), a letter of the early Cyrillic alphabet, adopted from Greek

Psychology or psychologist, denoted Ψ, used in the logo of various psychological associations
Psi Chi, the National Honor Society in Psychology
Psi-Theory, a theory about the human mind, a cognitive architecture

PSI (computational chemistry), software
PSI (prion), an infectious protein in yeast
Psi (instant messaging client), an XMPP client program
Pandemic severity index, by the US Centers for Disease Control and Prevention
Pounds per square inch, a unit of pressure
Photosystem I, a protein complex involved in photosynthesis
Pneumonia severity index
Pollutant Standards Index, an air pollution measure
Porous silicon, a material
Program-specific information, part of the MPEG transport stream protocol
Protein Structure Initiative, a structural genomics initiative of the U.S. NIGMS
J/psi meson, a subatomic particle
Water potential, denoted Ψ, in physical chemistry, the potential energy of a water solution relative to pure water
Wave function, denoted ψ, in quantum mechanics
'Yaw' angle, one of the Euler angles, denoted ψ in aerospace engineering
Potentially Shippable Increment, an acronym used in Scrum
Pre Shipment Inspection, also called Final Random Inspection (FRI)

Paul Scherrer Institute, Villigen, Switzerland
Poetry Slam, Inc.
Policy Studies Institute, a UK organization affiliated with the University of Westminster which undertakes research in matters pertaining to work and social policy and the environment
Population Services International, an international healthcare NGO based in Washington DC
Professional Secretaries International
Professors for a Strong Israel, a right-wing group of academics in Israel
Psychiatric Solutions, Inc., a corporation that owns psychiatric hospitals
Public Services International, a global union federation of public sector trade unions
Statisticians in the Pharmaceutical Industry

PSI, the psychic power used by some characters in the EarthBound series
Psi (comics), a character in DC Comics
Psi Corps, in the Babylon 5 fictional universe, the ruling body of Earth's telepaths
Psi Division, a division in the Judge Dredd and 2000 AD series of comics
Psi Factor: Chronicles of the Paranormal, a Canadian television series
Psi-Force, a comic series
Psi-Hawk, a comic character
Psi Lords, a comic series
Psi-Ops: The Mindgate Conspiracy, a video game in which characters use psychic powers

Partai Sosialis Indonesia ("Indonesian Socialist Party")
Italian Socialist Party (Partito Socialista Italiano)
Italian Socialist Party (2007)
United States Senate Homeland Security Permanent Subcommittee on Investigations, a sub-committee of the U.S. Senate
Proliferation Security Initiative, an international effort to interdict transfer of banned weapons and weapons technology
Public Sector Information, as in
- The European Union Directive on the re-use of public sector information (PSI directive), adopted in November 2003
- The United Kingdom's Office of Public Sector Information (OPSI), formerly Her Majesty's Stationery Office (HMSO)

Pechersk School International, in Kiev, Ukraine
Permanent staff instructor, in the British Army
PSI-20, a stock market index of companies that trade on Euronext Lisbon, the main stock exchange of Portugal
Psi Islands in the Melchior Islands, Antarctica
Presentence investigation report, an investigation into the history of person convicted of a crime
Pre-shipment inspection, a method of quality control
Professional Skills Institute, a private university in Toledo, Ohio
Pitch Shifter Industries, an acronym used by the British band Pitchshifter