I. ˈsī, ˈpsī noun
Etymology: Middle English, from Medieval Latin, from Late Greek, from Greek psei
Date: 15th century
: the 23d letter of the Greek alphabet — see alphabet table
II. ˈsī noun
Etymology: probably by shortening & alteration from psychic
Date: 1942
: parapsychological psychic phenomena or powers
III. abbreviation
pounds per square inch