A term used to describe the gradual movement caused by the wobble of the earth on its axis. The gradual turning of the earth's poles, much like that of a spinning top as it winds down, causes the seasons of the year to begin slightly earlier each year. Thus the exact moments of Spring and Autmnal equinoxes each year occurs a few minutes earlier than the year before. This gradual movement which takes 25,800 years to complete its cycle, causes the ages. (We are now in the Age of Aquarius.)
The intersection between the ecliptic and the celestial equator does not remain fixed with respect to the background constellations, but slowly move through them. Since this intersection is the position of the sun at the time of the equinoxes, this slow shift of position is called precession of the equinoxes.
The westward movement of the Vernal and Autumnal Equinoxes, relative to the orbit of the Earth, or the stars. The period of this movement is variously given as 21,000 or 26,000 years, leading to great confusion for students. The reason for this confusion is that the Vernal Equinox takes 26,000 years to move once around the sky, relative to the stars; but during that time, the orbit of the Earth is regressing (changing in the opposite direction), so it takes less time (only 21,000 years) for the Vernal Equinox to move once around the sky, relative to our orbit.