Is it ensure, insure, or assure?
23rd letter of the Greek alphabet. Use for "psychic force, paranormal phenomenon" dates from 1942 (probably from psychic (adj.)).
ψ The Greek letter psi. Entries beginning with this character are alphabetized under psi.
psi 1 (sī, psī)n. Symbol ψ The 23rd letter of the Greek alphabet.