Psi, PSI or Ψ may refer to:

Alphabetic letters

* Psi (Greek) (Ψ or ψ), the twenty-third letter of the Greek alphabet * Psi (Cyrillic), letter of the early Cyrillic alphabet, adopted from Greek

Arts and entertainment

* "Psi" as an abbreviation for psionics

Comics

* Psi (comics), DC Comics character * Psi Division, division in the '' Judge Dredd'' and ''2000 AD'' series of comics * ''

Psi-Force ''Psi-Force'' is a comic book series created by Archie Goodwin and Walt Simonson and published by Marvel Comics under their New Universe imprint from 1986 to 1989. It ran for 32 issues and an annual (October 1987). It concerned a group of ...

'', comic series *

Psi-Hawk ''Psi-Force'' is a comic book ongoing series, series created by Archie Goodwin (comics), Archie Goodwin and Walt Simonson and published by Marvel Comics under their New Universe imprint from 1986 in comics, 1986 to 1989 in comics, 1989. It ran for ...

, comic character * '' Psi Lords'', comic series

Music

* ''PSI'' (album), album by Pitchshifter (2002) * Psi Com, 1980s rock band * Psi Power, song by rock group Hawkwind (1978) * Psi Records, music record label * PSI Records, music record label * Ψ CMX DVD, Finnish language video album by the band CMX * ''Pitch Shifter Industries'', acronym used by the British band Pitchshifter * Logo used by the band Twenty One Pilots during their '' Scaled and Icy'' (2021) album era

Other uses in arts and entertainment

* ''Psi'' (TV series), Brazil * Psi Corps, in the ''Babylon 5'' fictional universe * '' Psi Factor: Chronicles of the Paranormal'', Canadian television series * '' Psi-Ops: The Mindgate Conspiracy'', 2004 video game

Organizations

Government and politics

* Italian Socialist Party (''Partito Socialista Italiano''), socialist and later social-democratic political party in Italy * Italian Socialist Party (2007), social-democratic political party in Italy * Indonesian Solidarity Party (''Partai Solidaritas Indonesia'') * Indonesian Socialist Party (''Partai Sosialis Indonesia'') * Proliferation Security Initiative, against weapons technology transfer * ''Public Sector Information'', as in: ** The 2003 EU Directive on the re-use of public sector information ** The UK Office of Public Sector Information (OPSI), formerly HMSO *

United States Senate Homeland Security Permanent Subcommittee on Investigations The Permanent Subcommittee on Investigations (PSI), stood up in March 1941 as the "Truman Committee," is the oldest subcommittee of the United States Senate Committee on Homeland Security and Governmental Affairs (formerly the Committee on Govern ...

Other organizations

Paul Scherrer Institute The Paul Scherrer Institute (PSI) is a multi-disciplinary research institute for natural and engineering sciences in Switzerland. It is located in the Canton of Aargau in the municipalities Villigen and Würenlingen on either side of the Ri ...

, research institute in Villigen, Switzerland *

Pechersk School International Pechersk School International (PSI) is a school located in Kyiv, Ukraine. It provides education ranging from the early childhood-level, to the end of the IB diploma-programme. PSI is a dynamic, diverse, not-for-profit international school community ...

, Kyiv, Ukraine * Physics Society of Iran * Planetary Science Institute, Tucson, Arizona, US * Poetry Slam, Inc., promoting poetry *

Policy Studies Institute The Policy Studies Institute (PSI) is a British think-tank and research institute. PSI began in 1931 as Political and Economic Planning and became the Policy Studies Institute in 1978 on its merger with the Centre for Studies in Social Policy (est ...

, UK research organization * Population Services International, global health organization * Professional Skills Institute, technical school in Maumee, Ohio, US * Prudential Securities Incorporated, financial services company * PSI Seminars, on awareness training * Public Services International, trades unions federation * Statisticians in the Pharmaceutical Industry

Parapsychology

* Psi (parapsychology), psychic or paranormal phenomena * Psi hit, parapsychological experimentation term * Paranormal Site Investigators, in the UK

Science, technology, and mathematics

Biology and medicine

* PSI (prion), infectious protein in yeast * Pandemic severity index, former US CDC influenza scale * Photosystem I, protein complex involved in photosynthesis * Pneumonia severity index * Protein Structure Initiative, of the U.S. NIGMS

Computing

* Psi (instant messaging client), XMPP client program * Potentially Shippable Increment, in the Scrum software development methodology * Program-specific information, part of the MPEG transport stream protocol * Pressure stall information, a Linux kernel patch for

out of memory Out of memory (OOM) is an often undesired state of computer operation where no additional memory can be allocated for use by programs or the operating system. Such a system will be unable to load any additional programs, and since many programs ...

management

Cryptography

* Private set intersection, a cryptographic technique to compute the intersection of two encrypted sets

Economics

* PSI-20, Lisbon, Portugal stock market index * Private sector involvement, in sovereign debt crisis resolution

Engineering

* "Yaw" angle, one of the Euler angles, denoted ψ in aerospace engineering * Present serviceability index, a pavement performance indicator introduced by AASHTO

Geography

* Psi Islands, in the Melchior Islands, Antarctica

Mathematics

* Chebyshev function * Dedekind psi function * Digamma function * Polygamma functions * Stream function, in two-dimensional flows * Polar tangential angle of a curve * Probability of ultimate ruin, in

ruin theory In actuarial science and applied probability, ruin theory (sometimes risk theory or collective risk theory) uses mathematical models to describe an insurer's vulnerability to insolvency/ruin. In such models key quantities of interest are the proba ...

* Supergolden ratio * Reciprocal Fibonacci constant * Population Stability Index ( Kullback–Leibler divergence#Symmetrised divergence)

Physics and chemistry

* Pound per square inch, unit of pressure * PSI (computational chemistry), software * J/psi meson, subatomic particle * Porous silicon ("PS" or "pSi"), a form of silicon with introduced nanopores in its microstructure *

Water potential Water potential is the potential energy of water per unit volume relative to pure water in reference conditions. Water potential quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure and mat ...

, denoted Ψ, in physical chemistry, the potential energy of a water solution relative to pure water *

Wave function In quantum physics, a wave function (or wavefunction) is a mathematical description of the quantum state of an isolated quantum system. The most common symbols for a wave function are the Greek letters and (lower-case and capital psi (letter) ...

, denoted ψ, in quantum mechanics

Psychology

Psychology Psychology is the scientific study of mind and behavior. Its subject matter includes the behavior of humans and nonhumans, both consciousness, conscious and Unconscious mind, unconscious phenomena, and mental processes such as thoughts, feel ...

or psychologist, denoted Ψ, used in the logo of various psychological associations * Psi Chi, the National Honor Society in psychology * Psi-theory, theory about the human mind, a cognitive architecture * Psychiatric Solutions, Inc., corporation that owns psychiatric hospitals

Other uses

* Parasocial interaction, non-reciprocal interactions, as between a fanbase and a performer * Permanent staff instructor, in the British Army * Personalized System of Instruction * Pollutant Standards Index, air pollution measure * Pre-shipment inspection, method of quality control * Presentence investigation report, investigation into the history of person convicted of a crime * Proto-Siouan language, the reconstructed ancestor of the Siouan languages