The letter **ψ** (psi) was yet another Greek addition to the alphabet, placed after the letters of Phoenician origin, along with φ, χ and ω. According to Wikipedia,

The letter’s origin is uncertain. It may or may not derive from the Phoenician alphabet. It appears in the 7^{th}century BC, expressing /ps/ in the Eastern alphabets, but /kʰ/ in the Western alphabets (the sound expressed by Χ in the Eastern alphabets).

The letter ψ has a number of uses in maths and sciences, such as:

- in biochemistry: pseudouridine
- in carotenoid nomenclature: acyclic end group. For example, lycopene is ψ,ψ-carotene.
- in hydrostatics and biology: water potential
- in fluid mechanics: stream function
- in mathematics: the division polynomials
*ψ*_{n} - in mathematics: the polygamma function
- in mathematics: the reciprocal Fibonacci constant with approximate value of 3.35989
- in molecular biology: the ψ packaging element
- in particle physics: the J/ψ meson (psion)
- in protein chemistry: one of three dihedral angles (
*φ*,*ψ*and*ω*) in a protein chain - in quantum mechanics: wave function

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