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), psychic or paranormal phenomena

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