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).
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 ψn
- 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