23rd letter of the Greek alphabet. Its use for "psychic force, paranormal phenomenon" dates from 1942 (probably from psychic (adj.)).
Entries linking to psi
psychic adj.
1872, "of or pertaining to the human soul" (earlier psychical, 1640s), from Greek psykhikos "of the soul, spirit, or mind" (opposed to somatikos), also (New Testament) "concerned with the life only, animal, natural," from psykhē "soul, mind, life" (see psyche).
The meaning "characterized by psychic gifts; pertaining to the of extraordinary and obscure phenomena of the mind not usually treated by psychologists" (mind-reading, second sight, etc.) is attested from 1871.
psionic adj.
"of or pertaining to psi in the 'paranormal' sense," 1952, from psi + ending from electronic, etc. First attested in science fiction. Related: Psionics.