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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").
Contents
For Greek loanwords in Latin and modern languages with Latin alphabets, psi is usually transliterated as "ps". In English, the letter is pronounced /ˈsaɪ/ or sometimes /ˈpsaɪ/. (In Greek, it is pronounced [ˈpsi].)
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 (). There were early graphical variants that omitted the stem ("chickenfoot-shaped psi" as: or ).
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".
Use as a symbol
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:
Psi is also used as the symbol for the polygamma function, defined by
where
The letters Ψ or ψ can also be a symbol for:
Character Encodings
These characters are used only as mathematical symbols. Stylized Greek text should be encoded using the normal Greek letters, with markup and formatting to indicate text style.