mean sun

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

• noun A hypothetical sun defined as moving at a uniform rate along the celestial equator at the mean speed with which the real sun apparently moves along the ecliptic, used in computing the mean solar day.

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

• noun a theoretical sun that moves along the celestial equator at a constant speed and completes its annual course in the same amount of time the real sun takes at variable speeds