from The American Heritage® Dictionary of the English Language, 5th Edition.

  • noun A region of the ionosphere, extending from about 90 to 150 kilometers (55 to 95 miles) above the earth and influencing long-distance communications by strongly reflecting radio waves in the range from one to three megahertz.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • noun a region of the ionosphere (from 50 to 90 miles up) that reflects radio waves of medium length


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