Psi, PSI or Ψ may refer to:

Alphabetic letters

Psi (Greek) Psi (uppercase , lowercase ; ''psi'' ) is the 23rd letter of the Greek alphabet and is associated with a Greek numerals, numeric value of 700. In both Classical Greek, Classical and Modern Greek, the letter indicates the combination (as i ...

(Ψ, ψ), the 23rd letter of the Greek alphabet *

Psi (Cyrillic) Psi (Ѱ, ѱ; italics: ) is a letter in the early Cyrillic alphabet, derived from the Greek letter psi (Ψ, ψ). It represents the sound /ps/, as in English ''naps''. According to the school rules developed in the 16th and the 17th centuries, suc ...

(Ѱ, ѱ), 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 Judge Joseph Dredd is a fictional character created by writer John Wagner and artist Carlos Ezquerra. He first appeared in the second issue of '' 2000 AD'' (1977), which is a British weekly anthology comic. He is the magazine's longest-running ...

'' and ''2000 AD'' series of comics * ''

Psi-Force ''Psi-Force'' was 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 adole ...

'', comic series *

Psi-Hawk ''Psi-Force'' was 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 adol ...

, comic character * ''

Psi Lords The Psi-Lords are a fictional team of superheroes from the Valiant Comics universe. Publication history The characters first appeared in the second volume of ''Rai and the Future Force #21'' after a cameo in ''Magnus #36'' and were featured in ...

'', comic series

Music

* ''PSI'' (album), album by Pitchshifter (2002) * Psi Com, 1980s rock band *

Psi Power Psi, PSI or Ψ may refer to: Alphabetic letters * Psi (Greek) (Ψ, ψ), the 23rd letter of the Greek alphabet * Psi (Cyrillic) (Ѱ, ѱ), letter of the early Cyrillic alphabet, adopted from Greek Arts and entertainment * "Psi" as an abbreviatio ...

, song by rock group Hawkwind (1978) * 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 Pitchshifter are an English industrial rock band from Nottingham, formed in 1989. The band was started by lead guitarist and programmer Johnny A. Carter, and bassist and vocalist Mark Clayden. The band’s early material was characterized for ...

* Logo used by the band

Twenty One Pilots Twenty One Pilots (stylized in all lowercase or as twenty øne piløts) are an American musical duo from Columbus, Ohio. Initially a band, the group was formed in 2009 by lead vocalist Tyler Joseph along with Nick Thomas and Chris Salih, who ...

during their ''

Scaled and Icy ''Scaled and Icy'' is the sixth studio album by American musical duo Twenty One Pilots. The album was released on May 21, 2021, through Fueled by Ramen and Elektra. The album's title is a play on "scaled back and isolated", a phrase frontman Ty ...

'' (2021) album era

Other uses in arts and entertainment

* ''Psi'' (TV series), Brazil *

Psi Corps The list of ''Babylon 5'' characters contains characters from the entire ''Babylon 5'' universe. The Babylon station was conceived as a political and cultural meeting place. As such, one of the show's many themes is the cultural and social intera ...

, 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 The Italian Socialist Party (, PSI) was a Socialism, socialist and later Social democracy, social-democratic List of political parties in Italy, political party in Italy, whose history stretched for longer than a century, making it one of the l ...

(''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 The Indonesian Solidarity Party ( id, Partai Solidaritas Indonesia, abbreviated PSI) is a political party in Indonesia that focuses on women's rights, pluralism, and Indonesian youth. It is led by Giring Ganesha. PSI endorsed incumbent Presiden ...

(''Partai Solidaritas Indonesia'') *

Indonesian Socialist Party The Socialist Party of Indonesia ( id, Partai Sosialis Indonesia) was a political party in Indonesia from 1948 until 1960, when it was banned by President Sukarno. Origins In December 1945 Amir Sjarifoeddin's Socialist Party of Indonesia (Pa ...

(''Partai Sosialis Indonesia'') *

Proliferation Security Initiative The Proliferation Security Initiative (PSI) is a global effort that aims to stop trafficking of weapons of mass destruction (WMD), their delivery systems, and related materials to and from states and non-state actors of proliferation concern. L ...

, against weapons technology transfer * ''Public Sector Information'', as in: ** The 2003 EU

Directive on the re-use of public sector information Directive 2003/98/EC on the re-use of public sector information, known as the PSI Directive,
...

** 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 Governme ...

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 River ...

, 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, 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 Public Services International (PSI) is the global union federation for workers in public services, including those who work in social services, health care, municipal services, central government and public utilities. , PSI has 700 affiliated ...

, trades unions federation * Statisticians in the Pharmaceutical Industry

Parapsychology

Psi (parapsychology) Parapsychology is the study of alleged psychic phenomena (extrasensory perception, telepathy, precognition, clairvoyance, psychokinesis (also called telekinesis), and psychometry) and other paranormal claims, for example, those related to near ...

, 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 Photosystem I (PSI, or plastocyanin–ferredoxin oxidoreductase) is one of two photosystems in the photosynthetic light reactions of algae, plants, and cyanobacteria. Photosystem I is an integral membrane protein complex that us ...

, 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

Economics

PSI-20 The PSI-20 (an acronym of Portuguese Stock Index) is a benchmark stock market index of companies that trade on Euronext Lisbon, the main stock exchange of Portugal. The index tracks the prices of the twenty listings with the largest market cap ...

, 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 The present serviceability index (PSI) is a pavement performance measure. Introduced by the American Association of State Highway and Transportation Officials (AASHTO), the PSI is one of the most widely used pavement performance indicators after pa ...

, a pavement performance indicator introduced by AASHTO

Geography

* Psi Islands, in the Melchior Islands, Antarctica

Mathematics

Chebyshev function In mathematics, the Chebyshev function is either a scalarising function (Tchebycheff function) or one of two related functions. The first Chebyshev function or is given by :\vartheta(x)=\sum_ \ln p where \ln denotes the natural logarithm, ...

* Dedekind psi function *

Digamma function In mathematics, the digamma function is defined as the logarithmic derivative of the gamma function: :\psi(x)=\frac\ln\big(\Gamma(x)\big)=\frac\sim\ln-\frac. It is the first of the polygamma functions. It is strictly increasing and strict ...

Polygamma function In mathematics, the polygamma function of order is a meromorphic function on the complex numbers \mathbb defined as the th derivative of the logarithm of the gamma function: :\psi^(z) := \frac \psi(z) = \frac \ln\Gamma(z). Thus :\psi^(z) ...

s *

Stream function The stream function is defined for incompressible ( divergence-free) flows in two dimensions – as well as in three dimensions with axisymmetry. The flow velocity components can be expressed as the derivatives of the scalar stream function. T ...

, 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 prob ...

Supergolden ratio In mathematics, two quantities are in the supergolden ratio if the quotient of the larger number divided by the smaller one is equal to :\psi = \frac which is the only real solution to the equation x^3 = x^2+1. It can also be represented using ...

Reciprocal Fibonacci constant The reciprocal Fibonacci constant, or ψ, is defined as the sum of the reciprocals of the Fibonacci numbers: :\psi = \sum_^ \frac = \frac + \frac + \frac + \frac + \frac + \frac + \frac + \frac + \cdots. The ratio of successive terms in this s ...

* Population Stability Index ( Kullback–Leibler divergence#Symmetrised divergence)

Physics and chemistry

Pound per square inch The pound per square inch or, more accurately, pound-force per square inch (symbol: lbf/in2; abbreviation: psi) is a unit of pressure or of stress based on avoirdupois units. It is the pressure resulting from a force of one pound-force applied t ...

, unit of pressure * PSI (computational chemistry), software * J/psi meson, subatomic particle *

Porous silicon Porous silicon (abbreviated as "PS" or "pSi") is a form of the chemical element silicon that has introduced nanopores in its microstructure, rendering a large surface to volume ratio in the order of 500 m2/cm3. History Porous silicon was disco ...

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

Wave function A wave function in quantum physics is a mathematical description of the quantum state of an isolated quantum system. The wave function is a complex-valued probability amplitude, and the probabilities for the possible results of measurements ...

, denoted ψ, in quantum mechanics

Psychology

Psychology Psychology is the scientific study of mind and behavior. Psychology includes the study of conscious and unconscious phenomena, including feelings and thoughts. It is an academic discipline of immense scope, crossing the boundaries between ...

or psychologist, denoted Ψ, used in the logo of various psychological associations *

Psi Chi Psi Chi () is a college student honor society in psychology with international outreach founded in 1929 at the University of Kansas in the United States. Psi Chi is one of the largest honor societies in the United States, with more than 1,150 cha ...

, 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 Parasocial interaction (PSI) refers to a kind of psychological relationship experienced by an audience in their mediated encounters with performers in the mass media, particularly on television and on online platforms. Viewers or listeners com ...

, non-reciprocal interactions, as between a fanbase and a performer * Permanent staff instructor, in the British Army * Personalized System of Instruction *

Pollutant Standards Index The Pollutant Standards Index (PSI) is a type of air quality index, which is a number used to indicate the level of pollutants in air. Initially PSI was based on five air pollutants, but since 1 April 2014 it has also included fine particulat ...

, air pollution measure *

Pre-shipment inspection Pre-shipment inspection (also preshipment inspection or PSI) is a part of supply chain management and an important quality control method for checking the quality of goods clients buy from suppliers. PSI helps ensure that production complies with ...

, method of quality control * Presentence investigation report, investigation into the history of person convicted of a crime