For other uses, see Psi.

Psi (uppercase ?, lowercase ?) 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/ (like in English "lapse"). In Greek, this consonant cluster can occur in the syllable initial position, as in the Greek word "????" [psári (=fish)]. However, in some languages (including English) this combination is not possible at the beginning of a syllable. In Latin, Greek words beginning with psi are transcribed by ps-, but seem to have been pronounced simply as s-, with a quiescent p. This pronunciation has affected that of Greek loanwords beginning with the letter in several languages. In English, for example, psychology is pronounced with a silent p, and the name of the letter is often pronounced ("sigh"), although this is not always the case. The letter was adopted into the Old Italic alphabet, and its shape is continued into the Algiz rune of the Elder Futhark.

The letter may have originated from the practice of writing the sigma over the pi, eventually making the combination into a single letter. Psi was also adopted into the early Cyrillic alphabet. See psi (Cyrillic) (, ). This may have also been since the letter, both in lower case and uppercase form, resembles the trident wielded by Poseidon, the Greek god of water/ocean.

The letter psi is commonly used in physics for representing a wavefunction in quantum mechanics, particularly with the Schrödinger equation and bra-ket notation: \langle\phi|\psi\rangle. It is also used to represent the (generalized) positional states of a qubit in a quantum computer.

Psi is also used as the symbol for the polygamma function, defined by

\psi_n(x) = \frac{d^{(n)}}{dx^{(n)}}\frac{\Gamma '(x)}{\Gamma (x)}\,\!

where \Gamma (x) is the gamma function.

The letters ? or ? can also be a symbol for:

• psychology, psychiatry, and sometimes parapsychology (involving paranormal or relating with the supernatural subjects).
• In mathematics, the number of the "Supergolden Ratio", approximately 1.4 and the solution to the cubic equation x³ = x² + 1.
• Water potential in movement of water between plant cells.
• Stream function in fluid mechanics defining the curve to which the flow velocity is always tangent.
• One of the dihedral angles in the backbones of proteins
• The planet Neptune
• The Schrodinger Equation and throughout quantum mechanics, ?(x) stands for the wave function
• Indiana University (as a superimposed I and U)
• Gangster Disciples
• Crips
• A Sai, the name of which is pronounced the same way.
• Pharmacology, general pharmacy
• In virology the ? site is a viral packaging signal.
• The J/? meson, in particle physics.
• In the manga Monochrome, Psi is a mentally disturbed character, though her name is usually spelled out.

als:? ar:??? (???) arc:? br:Psi (lizherenn) bg:??? ca:Psi cs:Psí (písmeno) cy:Psi (llythyren) da:Psi (bogstav) de:Psi el:?? es:? eo:Psio (litero) eu:Psi fr:Psi (lettre grecque) gd:Psi gl:Psi ko:? hr:Psi (slovo) is:Psí it:Psi (lettera) he:??? ka:??? (???) ht:? ku:Psi la:Psi lt:Psi (raid?) hu:Pszí ms:Psi nl:Psi (letter) ja:? no:Psi nn:Psi nds:Psi pl:Psi (litera) pt:? ru:??? (?????) simple:Psi (letter) sk:Psí (grécke písmeno) sl:Psi sh:Psi (slovo) fi:Psii sv:Psi th:??? tr:? uk:??? (??????) zh:?