Psi has multiple meanings:

• Psi (letter) (Ψ, ψ) of the Greek alphabet
• Psi (Cyrillic) (Ѱ, ѱ), letter of the early Cyrillic alphabet, adopted from Greek
• Psi (instant messenger), the popular Jabber client program
• J/ψ particle, a subatomic particle
• Schrödinger equation in quantum mechanics, ψ
• Ψ in mathematics is the angle between the tangent and the x-axis in the intrinsic coordinates system
• Psi denotes the unknown factor in parapsychology, usually referring to psychic or psionic powers
• Psi, a character in the game and television series Pokémon

PSI stands for:

• PSI (computational chemistry) software
• Italian Socialist Party (Partito Socialista Italiano)
• Paul Scherrer Institute in Villigen, Switzerland
• Permanent Staff
  
  
  
  Instructor in the British Army
• Permanent Subcommittee on Investigations (PSI) of the U.S. Senate
• Population Services International, an international healthcare NGO based in Washington D.C.
• pound-force per square inch (psi), a non-SI unit of pressure
• Professional Secretaries International
• Professors for a Strong Israel
• Program Specific Information, metadata for digital transmission
• Proliferation Security Initiative
• Public Services International, a global union federation of public sector trade unions
• Socialist Party of the Islands (Partit Socialista de les Illes)

Psi (Begriffsklärung) Psi Psi PSI Psi PSI Psi Psi Psi Psi (olika betydelser)