1. Psi (Ψ ψ) is a letter of the Greek alphabet. See Psi (letter).
2. The letter Psi was also used in the Early Cyrillic alphabet. See Psi (Cyrillic).
3. In mathematics (Ѱ, ѱ) is the angle between the tangent and the x-axis in the intrinsic coordinates system.
4. The lower-case letter ψ is used as the symbol for the wave function in quantum mechanics.
5. In high-energy physics, Ψ refers to a meson also called the J/Ψ particle
6. It has come to be a symbol for psychology-related topics, and has also been used as a shorthand for supposed psychic abilities or powers.
7. Psi is also an open-source instant messenger. See Psi (instant messenger).
8. Psi was also a character on the popular game and television series, Pokémon.

The abbreviation PSI stands for

1. Pounds per square inch (PSI).
2. The Paul Scherrer Institute in Villigen , Switzerland