precession of the equinoxes

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

• noun A slow westward shift of the equinoxes along the plane of the ecliptic, resulting from precession of the earth's axis of rotation, and causing the equinoxes to occur earlier each sidereal year. The precession of the equinoxes occurs at a rate of 50.27 seconds of arc a year; a complete precession requires 25,800 years.

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

• noun The slow westward shift of the equinoxes along the plane of the ecliptic, resulting from precession of the earth's axis of rotation, and causing the equinoxes to occur earlier each year

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

• noun a slow westward shift of the equinoxes along the plane of the ecliptic caused by precession of the Earth's axis of rotation