precession of the equinoxes

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. 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.

Wiktionary

1. n. 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

GNU Webster's 1913

1. n. (Astron.) the slow backward motion of the equinoctial points along the ecliptic, at the rate of 50.2″ annually, caused by the action of the sun, moon, and planets, upon the protuberant matter about the earth's equator, in connection with its diurnal rotation; -- so called because either equinox, owing to its westerly motion, comes to the meridian sooner each day than the point it would have occupied without the motion of precession, and thus precedes that point continually with reference to the time of transit and motion.

WordNet 3.0

1. n. a slow westward shift of the equinoxes along the plane of the ecliptic caused by precession of the Earth's axis of rotation