Meaning of hypoionian mode

hy'po•i•o'ni•an mode'

Pronunciation: (hī'pō-ī-ō'nē-un, hip'ō-, hī"pō-, hip"ō-), [key]
— Music. Music.
  1. a plagal church mode represented on the white keys of a keyboard instrument by an ascending scale from G to G, with the final on C.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.