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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

See also

• Psy (disambiguation)
• PS1 (disambiguation)

• Disambiguation pages