Definition of "io"

io¹

/ˈaɪəʊ/

noun
plural ios
1. A type of moth, the io moth.
  Quotations
  These lines appear to serve as roadways or guides to any stragglers that may have hung back for some reason known only to an io.
  1936, Paul Griswold Howes, Hand book for the curious

io²

interjection
1. (rare) An exclamation of joy or triumph.
  Quotations
  Do as thou wilt, as a great god can, O Pan! Io Pan! Io Pan! Io Pan Pan! I am awake In the grip of the snake.
  1913, Crowley, Aleister, “Hymn To Pan”, in Book 4‎, University of California Libraries, Magick in Theory and Practice