Dictionary:E-layer

1. The Heaviside layer, a layer of very thin air about 110 km high in the ionosphere that is ionized by sun light and that reflects high-frequency (HF) radio waves back to the earth during the daytime and medium frequency (MF) waves at night. See Figure A-22 and also F-layer and D-layer. 2. A layer within the Earth; see Figure E-1.