Meaning of extraordinary wave

extraor'dinary wave"

Pronunciation: [key]
— Radio. Radio.
  1. (of the two waves into which a radio wave is divided in the ionosphere under the influence of the earth's magnetic field) the wave with characteristics different from those that the undivided wave would have exhibited in the absence of the magnetic field. Also calledCf. ordinary wave.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.