also: プシー
noun
The 23rd letter of the Greek alphabet (Ψ, ψ). Used as a symbol in physics, mathematics, psychology, and other sciences.
Ψはギリシャ文字の23番目の文字です。
Psi is the 23rd letter of the Greek alphabet.
量子力学では、ψは波動関数を表す。
In quantum mechanics, ψ represents the wave function.
From Greek ψεῖ (pseî), via English 'psi'. The Japanese readings プサイ and プシー reflect different English pronunciations.