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psi

or p.s.i.

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  1. pounds per square inch.

psi1

[sahy, psahy]
noun, plural psis.
  1. the 23rd letter of the Greek alphabet (Ψ, ψ).
  2. the group of consonant sounds represented by this letter.

Origin of psi1

1350–1400; Middle English < Greek pseî

psi2

[sahy]
noun
  1. any purportedly psychic phenomenon, as psychokinesis, telepathy, clairvoyance, or the like.

Origin of psi2

1940–45; shortening of psychic or parapsychic
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

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British Dictionary definitions for psi

psi1

abbreviation for
  1. pounds per square inch

psi2

noun
  1. the 23rd letter of the Greek alphabet (Ψ, ψ), a composite consonant, transliterated as ps
    1. paranormal or psychic phenomena collectively
    2. (as modifier)psi powers
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for psi

n.

23rd letter of the Greek alphabet. Use for "psychic force, paranormal phenomenon" dates from 1942 (probably from psychic (adj.)).

Online Etymology Dictionary, © 2010 Douglas Harper

psi in Medicine

psi

(sī, psī)
n.
  1. The 23rd letter of the Greek alphabet.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.