Source : Webster's Revised Unabridged Dictionary (1913)

Precession \Pre*ces"sion\, n. [L. praecedere, praecessum, to go
   before: cf. F. pr['e]cession. See {Precede}.]
   The act of going before, or forward.

   {Lunisolar precession}. (Astron.) See under {Lunisolar}.

   {Planetary precession}, that part of the precession of the
      equinoxes which depends on the action of the planets
      alone.

   {Precession of the equinoxes} (Astron.), the slow backward
      motion of the equinoctial points along the ecliptic, at
      the rate of 50.2[sec] 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.

Source : WordNet®

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