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, but in the later part of the 20th century found along the following line of thought:

"The term psi denotes anomalous processes of information or energy transfer, processes such as telepathy or other forms of extrasensory perception that are currently unexplained in terms of known physical or biological mechanisms. The term is purely descriptive: It neither implies that such anomalous phenomena are paranormal nor connotes anything about their underlying mechanisms."

--Daryl J. Bem and Charles Honorton*

• Psi, a character in the game and television series Pokémon
• Psi (comics), a DC Comics character

A homophone of Psi is Sai, which is an Asian weapon shaped (probably coincidently) like the Greek letter Psi.

PSI stands for:

• pound-force per square inch (psi), a non-SI unit of pressure
• 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.
• 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)
• People with Speech Impairment
• Pechersk School International, a school in Kiev
• Protein Synthesis Inhibition

Psi | Psi (Begriffsklärung) | Psi | PSI | Psi | PSI | PSI | Psi | PSI | Psi | Psi | Psi (значения) | Psi | Psi (olika betydelser)