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. In both Classical and Modern Greek, the letter indicates the combination /ps/ (as in English word "lapse").

For Greek loanwords in Latin and modern languages with Latin alphabets, psi is usually transliterated as "ps".

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 ⟨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.[2] However, it was abandoned after his death.

The classical Greek letter was adopted into the early Cyrillic alphabet as "Ѱ".

Use as a symbol

The symbol Ψ or ψ has many uses across different academic and informal contexts:

Unicode

  1. The MATHEMATICAL characters are used in math. Stylized Greek text should be encoded using the normal Greek letters, with markup and formatting to indicate text style.

See also

Look up Ψ or ψ in Wiktionary, the free dictionary.

Notes and references

  1. "psi". The Chambers Dictionary (9th ed.). Chambers. 2003. ISBN 0-550-10105-5.
  2. Oliver, Revilo P. (1949). "The Claudian Letter Ⱶ". American Journal of Archaeology. 53 (3): 249–257. doi:10.2307/500662. JSTOR 500662. S2CID 193082268.
  3. Weisstein, Eric W. "Polygamma Function". mathworld.wolfram.com. A special function mostly commonly denoted ψ_n(z), ψ^((n))(z), or F_n(z-1)...
  4. "About - The Parapsychological Association". www.parapsych.org. The Parapsychological Association, Inc. (PA) is the international professional organization of scientists and scholars engaged in the study of 'psi' (or 'psychic') experiences, such as telepathy, clairvoyance, remote viewing, psychokinesis, psychic healing, and precognition.
  5. Weisstein, Eric W. "Reciprocal Fibonacci Constant". mathworld.wolfram.com.
  6. Weisstein, Eric W. "Chebyshev Functions". mathworld.wolfram.com.
  7. WELBAUM, GREGORY E.; BRADFORD, KENT J. (1991-03-01). "Water Relations of Seed Development and Germination in Muskmelon (Cucumis melo L.): VI. INFLUENCE OF PRIMING ON GERMINATION RESPONSES TO TEMPERATURE AND WATER POTENTIAL DURING SEED DEVELOPMENT". Journal of Experimental Botany. 42 (3): 393–399. doi:10.1093/jxb/42.3.393. ISSN 0022-0957. Germination sensitivities to temperature and water potential (ψ) were quantified as indicators of the influence of seed maturity and priming on seed vigour.
  8. IUPAC-IUB Commission on Biochemical Nomenclature (1970). "Abbreviations and symbols for nucleic acids, polynucleotides, and their constituents". Biochemistry. 9 (20): 4022–4027. doi:10.1021/bi00822a023.
  9. "Torsion Angles and the Ramachnadran Plot in Protein Structures". proteinstructures.com. These angles describe the rotations of the polypeptide backbone around the bonds between N-Cα (called Phi, φ) and Cα-C (called Psi, ψ) and essentially determine the fold of the protein.
  10. Although the university itself refers to its logo as a trident, not the Greek letter psi: "The IU trident—the only logo at Indiana University". Indiana University. At IU, the trident is the only logo we use, both institution-wide and at the unit level.
  11. Rein, Alan (2020-07-28). "The heart of the HIV RNA packaging signal?". Proceedings of the National Academy of Sciences of the United States of America. 117 (33). doi:10.1073/pna (inactive 18 July 2025). PMC 7443930. vRNA is selectively packaged because it contains a "packaging signal" or "ψ" [also designated the "core encapsidation signal" (2)].{{cite journal}}: CS1 maint: DOI inactive as of July 2025 (link)
  12. "J/psi particle | Quark, Charm & Meson | Britannica". www.britannica.com. J/psi particle, type of meson consisting of a charmed quark and a charmed antiquark.
  13. Srivastava, Manishi; Varma, Vishwanath; Abhilash, Lakshman; Sharma, Vijay Kumar; Sheeba, Vasu (June 2019). "Circadian Clock Properties and Their Relationships as a Function of Free-Running Period in Drosophila melanogaster". Journal of Biological Rhythms. 34 (3): 231–248. doi:10.1177/0748730419837767. ISSN 1552-4531. PMID 30939971. ... 2) accuracy (i.e., day-to-day stability of the phase relationship (ψ), where ψ is the duration between a phase of the rhythm and a phase of the external cycle) is greater for clocks with τ close to 24 h; ...
  14. Buchholz, W. (1986). "A new System of proof-theoretic ordinal functions" (PDF). Ann. Pure Appl. Logic. 32 (3): 195–207. doi:10.1016/0168-0072(86)90052-7.
  15. Gesenius' Hebrew Grammar
  16. Unicode Code Charts: Greek and Coptic (Range: 0370-03FF)