a dense layer of ions in the E region of the ionosphere at an altitude of c. 100 km (c. 62 mi), capable of reflecting shortwave radio waves
See E layer in American Heritage Dictionary 4
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. Also called E region, Heaviside layer, Kennelly-Heaviside layer.