"psi" = 23rd letter of the Greek alphabet Ψ or ψ which among others can be a symbol for psychology, psychiatry, and sometimes parapsychology involving paranorma

"psi" = 23rd letter of the Greek alphabet Ψ or ψ which among others can be a symbol for psychology, psychiatry, and sometimes parapsychology involving paranormal or relating with the supernatural subjects, especially research into extrasensory perception. Psi is not an initialism, so all the letters of the word "psi" should not be capitalized to prevent confusing it with an initialism such as pound per square inch, except when used at the beginning of a sentence and then only capitalizing the first letter.