# Psi

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, comic series
* Psi-Hawk, 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

### Other organizations

* Paul Scherrer Institute, research institute in Villigen, Switzerland
* Pechersk School International, 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, 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 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
* 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
* 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

### Psychology

* Psychology 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