PSI
UK[saɪ]US[saɪ]
n
The twenty-third letter of the Greek alphabet (Ψ, ψ)
n
A supposed psychic or paranormal faculty or phenomenon
n
A unit of pressure equal to one pound per square inch
Etymology
The word 'psi' entered English in the late 19th century as a straightforward transliteration of the name of the 23rd letter of the Greek alphabet, ψεῖ (pseî). Its use in physics to denote a unit of pressure (pound per square inch) is a modern, arbitrary abbreviation derived from the initial letters of the phrase. In the mid-20th century, parapsychologists adopted the symbol Ψ and the term 'psi' as a neutral, non-theoretical label for psychic and paranormal phenomena, drawing on the letter's historical association with the psyche and the mind in Greek culture. Thus, a single ancient letter name has evolved to serve distinct technical functions in modern scientific and pseudoscientific lexicons.
Analysis
The word 'psi' is a direct borrowing of the name of the Greek letter Ψ (ψ). It is not productively formed from smaller English morphemes.
Examples
The Greek letter psi is often used as a symbol in psychology.
Researchers conducted experiments to test for evidence of psi.
The tire pressure should be inflated to 32 psi.