I. noun Etymology: Middle English, from Medieval Latin, from Late Greek, from Greek psei Date: 15th century the 23d letter of the Greek alphabet, II. noun Etymology: probably by shortening & alteration from psychic Date: 1942 parapsychological psychic phenomena or powers, III. abbreviation pounds per square inch
PSI
Meaning of PSI in English
Merriam Webster. Explanatory English dictionary Merriam Webster. Толковый словарь английского языка Мерриам-Уэбстер. 2012