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 GreekArts and entertainment
* "Psi" as an abbreviation for psionicsComics
* 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 seriesMusic
* ''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 eraOther 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 gameOrganizations
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 InvestigationsOther 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 *Parapsychology
* Psi (parapsychology), psychic or paranormal phenomena * Psi hit, parapsychological experimentation term * Paranormal Site Investigators, in the UKScience, 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. NIGMSComputing
* 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 forCryptography
* Private set intersection, a cryptographic technique to compute the intersection of two encrypted setsEconomics
* PSI-20, Lisbon, Portugal stock market index * Private sector involvement, in sovereign debt crisis resolutionEngineering
* "Yaw" angle, one of the Euler angles, denoted ψ in aerospace engineering * Present serviceability index, a pavement performance indicator introduced by AASHTOGeography
* Psi Islands, in the Melchior Islands, AntarcticaMathematics
* 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 ("PS" or "pSi"), a form of silicon with introduced nanopores in its microstructure * Water potential, denoted Ψ, in physical chemistry, the potential energy of a water solution relative to pure water *Psychology
*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 languagesSee also
* Psy (disambiguation) * PS1 (disambiguation) {{disambiguation