Ψ stands for psychology and psychologist, often used in the logo of various psychological associations -- for example, see IUPsyS.

Italian Socialist Party (Partito Socialista Italiano)

Porous silicon

Presentence Investigation

Permanent Subcommittee on Investigations (PSI) of the US Senate

Pandemic Severity Index by the US Centers for Disease Control and Prevention

J/ψ particle, a subatomic particle

Paul Scherrer Institute in Villigen, Switzerland

PSI (prion), an infectious protein in yeast

Pneumonia severity index

Pounds per square inch, a unit of pressure

Proliferation Security Initiative

Psi (instant messaging client), a popular Jabber client program

Psi Power A 1978 song by the UK rock group Hawkwind.

Postpartum Support International, 501(c)3 organization devoted to women's mental health issues

Psi (Greek) (Ψ, ψ) a letter of the Greek alphabet

Psi (parapsychology)

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.

Professional Secretaries International

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

Psi (Cyrillic) (Ѱ, ѱ), a letter of the early Cyrillic alphabet, adopted from Greek

PSI Records, a record label

Public Services International, a global union federation of public sector trade unions

Permanent Staff Instructor in the British Army

Psi (comics), a character in DC Comics

PSI (computational chemistry) software

Professors for a Strong Israel

Wavefunction in Quantum Mechanics, ψ

In mathematics, Ψ is used to denote the angle between the tangent and the x-axis in the intrinsic coordinates system

Again in mathematics, Ψ is also used to denote the Digamma and Polygamma functions.

In physical chemistry, Ψ represents water potential, the potential energy of a water solution relative to pure water.

Pechersk School International in Kyiv, Ukraine.

Program Specific Information, part of the MPEG transport stream protocol. Metadata for digital transmission