What is the meaning of ψ?

Psychology.

Psychiatry.

Parapsychology, psychics.

A wavefunction.

Electric flux.

Water potential in cells.

Neptune.

asteroid (16) Psyche (a stylized 𝜓)

The Psalms (a book in the Bible).

a voiceless retroflex click, IPA k͜𝼊.

A bilabial trill, IPA ʙ.

Lower-case psi (ψεῖ), the 23rd letter of the ancient Greek alphabet. It represented the consonant cluster /ps/ in the Eastern Greek alphabet and the voiceless aspirated velar plosive /kʰ/ in the Western Greek alphabet (compare Etruscan 𐌙 khe). It is preceded by χ and followed by ω.

The lower case letter psi (ψι), the 23rd letter of the modern Greek alphabet.

Source: wiktionary.org