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
• Psi Factor: Chronicles of the Paranormal, 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

• 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