solar constant

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

• noun The average density of solar radiation measured outside Earth's atmosphere and at Earth's mean distance from the sun, equal to 0.140 watt per square centimeter.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The amount of incoming solar radiation per unit area, measured on the outer surface of Earth's atmosphere, in a plane perpendicular to the rays.

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

• noun the rate at which radiant solar energy is received at the outer layer of the earth's atmosphere