Definition of "psi"

psi¹

noun
countable and uncountable, plural psis
1. (countable) The twenty-third letter of Classical and Modern Greek and the twenty-fifth letter of Old and Ancient Greek.
2. (uncountable, parapsychology, science fiction) A form of psychic energy.
  Quotations
  ‘Come, lad,’ he said. ‘We will take tea together and speak of the noumenon, the psi and other more heterogenous phenomena.’
  1993, Will Self, My Idea of Fun
  When an event is classified as a psi phenomenon, it is claimed that all known channels for the apparent interaction have been eliminated.
  1996, Michael F. Stoeber, Hugo Anthony Meynell, Critical Reflections on the Paranormal (page 60)
  It traces the growth of homo gestalt with the uniting of six lovely outcasts of society who have psi powers and come together as a hive mind, thus creating a gestalt super-being.
  2005, Michael Ashley, Transformations: The History of the Science Fiction Magazine 1950 to 1970, Liverpool: Liverpool University Press, page 133

psi²

symbol
1. Pounds per square inch (an imperial unit of pressure)