Psi

Psi could mean:

As a word

• Psi (Cyrillic) (Ѱ, ѱ), a letter of the early Cyrillic alphabet, adopted from Greek
• Psi (Ψ, ψ) a letter of the Greek alphabet
• Psi (comics), a character in DC Comics
• Psi (instant messaging client), a popular Jabber client program
• Psi (parapsychology)

As a Greek letter

• J/ψ particle, a subatomic particle
• 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.
• Ψ stands for psychology and psychologist, often used in the logo of various psychological associations -- for example, see IUPsyS.

psi

• Pounds per square inch, a unit of pressure