Psi (Greek)

Psi /ˈ(p)saɪ, ˈ(p)siː/ (P)SY, (P)SEE[1] (uppercase Ψ, lowercase ψ or 𝛙; Greek: ψι psi [ˈpsi]) is the twenty-third and penultimate letter of the Greek alphabet and is associated with a numeric value of 700.

[citation needed] The Western letter (expressing /kʰ/, later /x/) was adopted into the Old Italic alphabets, and its shape is also continued into the Algiz rune <ᛉ> of the Elder Futhark.

Psi, or its Arcadian variant or was adopted in the Latin alphabet in the form of "Antisigma" (Ↄ, ↃC, or 𐌟) during the reign of Emperor Claudius as one of the three Claudian letters.

[citation needed] The classical Greek letter was adopted into the early Cyrillic alphabet as "Ѱ".

The letter psi is commonly used in physics to represent wave functions in quantum mechanics, such as in the Schrödinger equation and bra–ket notation:

The Greek alphabet on a black-figure pottery vessel, with an archaic chickenfoot-shaped psi.