celestial equator

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

• noun A great circle on the celestial sphere in the same plane as the earth's equator.

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

• noun astronomy A great circle on the celestial sphere, coincident with the plane of earth's equator.

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

• noun the great circle on the celestial sphere midway between the celestial poles