Psi

Psi has multiple meanings:

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• Psi (letter) (?, ?) of the Greek alphabet
• Psi (Cyrillic) (?, ?), letter of the early Cyrillic alphabet, adopted from Greek
• Psi (instant messenger) software
• Psi is supposed psychic or psionic powers in parapsychology
• Psi, a character in the game and television series Pokémon
• Schrödinger equation in quantum mechanics, ?
• J/? particle, a subatomic particle
• ? in mathematics is the angle between the tangent and the x-axis in the intrinsic coordinates system

The abbreviation PSI stands for:

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• Pound-force per square inch, a unit of pressure
• 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 Angolan company
• Proliferation Security Initiative
• Partito Socialista Italiano, the Italian Socialist Party