Psi

Psi may refer to:

As a word

* Psi (Greek) (Ψ, ψ) a letter of the Greek alphabet
* Psi (Cyrillic) (Ѱ, ѱ), a letter of the early Cyrillic alphabet, adopted from Greek
* Psi (instant messaging client), a popular Jabber client program
* Psi (parapsychology)
* Psi-Theory, a theory about the human mind, a cognitive architecture
* Psi hit, a parapsychological experimentation term
* "", a Canadian television series

As a Greek letter

* J/ψ particle, a subatomic particle
* Wavefunction in Quantum Mechanics, ψ
* In physical chemistry, Ψ represents water potential, the potential energy of a water solution relative to pure water
* Ψ stands for psychology and psychologist, often used in the logo of various psychological associations
* ΨΧ (Psi Chi), National Honor Society in Psychology

In mathematics

* ψ is used to denote tangential angle of a curve.
* ψ can also denote the digamma function, the polygamma functions, or the Dedekind psi function

As an abbreviation

PSI

In Computing:
* Program Specific Information, part of the MPEG transport stream protocol. Metadata for digital transmission
* PSI (computational chemistry) software

PSI can be an abbreviation for:
* Italian Socialist Party ("Partito Socialista Italiano")
* Pandemic Severity Index by the US Centers for Disease Control and Prevention
* Paul Scherrer Institute in Villigen, Switzerland
* Pechersk School International in Kyiv, Ukraine
* Permanent Staff Instructor in the British Army
* Permanent Subcommittee on Investigations (PSI) of the US Senate
* Pneumonia severity index
* Pollutant Standards Index, an air pollution index developed by the US Environmental Protection Agency
* Population Services International, an international healthcare NGO based in Washington DC
* Portuguese Stock Index (PSI-20), a benchmark stock market index of companies that trade on Euronext Lisbon, the main stock exchange of Portugal
* Presentence Investigation
* Professional Secretaries International
* Professors for a Strong Israel
* Proliferation Security Initiative
* Protein Structure Initiative, a structural genomics initiative of the U.S. NIGMS
* Public Services International, a global union federation of public sector trade unions
* PSI (prion), an infectious protein in yeast
* PSI Records, a record label
* Physique, Sciences de l'Ingenieur, a type of preparatory classes which emphasize on engineer science and physics in the french education system
* Statisticians In The Pharmaceutical Industry, the PSI acronym (rather than SPI) is chosen for its relation to the Greek letter Ψ, commonly used in statistical formulae

psi

* Pounds per square inch, a unit of pressure

pSi

* Porous silicon

In comics

* Psi (comics), a character in DC Comics
* "Psi-Force", a comic series
* Psi-Hawk, a comic character
* "Psi Lords", a comic series
* Psi Division, a division in the Judge Dredd and 2000 AD series of comics

In music

* Psi Power, a 1978 song by the UK rock group Hawkwind
* Psi Com, a 1980s rock band

In fiction

* In Babylon 5 universe, Psi Corps is the ruling body of Earth's telepaths
* PSI is the ability used by characters from the EarthBound series