psi

UK: /psaɪ/ US: (Greek letter sī, psī; Psych. sī)

Inflections of 'psi' (n): npl: psis

WordReference Random House Unabridged Dictionary of American English © 2026

psi¹  (sī, psī), n., pl. psis. 

1. Linguistics the 23rd letter of the Greek alphabet (Ψ, ψ).
2. Linguistics the group of consonant sounds represented by this letter.

• Greek pseî
• Middle English 1350–1400

psi²  (sī), n. 

1. Psychology, Holistic Therapy any purportedly psychic phenomenon, as psychokinesis, telepathy, clairvoyance, or the like. Cf. pseudoscience, parapsychology. 

• shortening of psychic or parapsychic 1940–45

psi, 

1. Physics pounds per square inch.

Also, p.s.i. 

Collins Concise English Dictionary © HarperCollins Publishers::

psi abbreviation

1. pounds per square inch

psi / psaɪ/ noun

1. the 23rd letter of the Greek alphabet (Ψ, ψ), a composite consonant, transliterated as ps
2. paranormal or psychic phenomena collectively