from The American Heritage® Dictionary of the English Language, 4th Edition
- n. A region of the earth's atmosphere where ionization caused by incoming solar radiation affects the transmission of radio waves. It extends from a height of 70 kilometers (43 miles) to 400 kilometers (250 miles) above the surface.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. The part of the Earth's atmosphere beginning at an altitude of about 50 kilometers (31 miles) and extending outward 500 kilometers (310 miles) or more.
- n. The similar region of the atmosphere of another planet.
from the GNU version of the Collaborative International Dictionary of English
- n. the outer region of the earth's atmosphere, beyond which is the
exosphere; it contains a high concentration of free electrons and ions, and extends from about 50 miles to about 250 miles above the earth's surface, but the height shows daily and seasonal variation, and the composition of the particles in the ionosphere is also affected by events such as solar flares.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. the outer region of the Earth's atmosphere; contains a high concentration of free electrons
iono- + -sphere (Wiktionary)