The Greek capital letter psi is often used to represent the word, or study of, Psychology. For example: Ψ = Psychology Ψist = Psychologist. Ψ, in biological terms, is a symbol used to represent water potential. Ψ, in astrology, is the symbol that represents Neptune. Ψ, in physics, is the symbol used to represent the quantum-mechanical wave-function.

Psi (uppercase Ψ, lowercase ψ; Greek: ψι psi) is the 23rd letter of the Greek alphabet and has a numeric value of 700. In both Classical and Modern Greek, the letter indicates the combination /ps/ (as in English word "lapse").

The letter's origin is uncertain. It may or may not derive from the Phoenician alphabet. It appears in the 7th century BC, expressing /ps/ in the Eastern alphabets, but /kʰ/ in the Western alphabets (the sound expressed by Χ in the Eastern alphabets). In writing, the early letter appears in an angular shape (Greek Psi straight.svg). There were early graphical variants that omitted the stem ("chickenfoot-shaped psi" as: Greek Psi V-shaped.svg or Greek Chi 05.svg).

The Western letter (expressing /kʰ/, later /x/) was adopted into the Old Italic alphabets, and its shape is also continued into the Algiz rune of the Elder Futhark. The classical Greek letter was adopted into the early Cyrillic alphabet as "Ѱ".

Source: Psi (letter) from Wikipedia

This Icon is available to order as a Decal or on a T-Shirt