Source : Webster's Revised Unabridged Dictionary (1913)

Horizon \Ho*ri"zon\, n. [F., fr. L. horizon, fr. Gr. ? (sc. ?)
   the bounding line, horizon, fr. ? to bound, fr. ? boundary,
   limit.]
   1. The circle which bounds that part of the earth's surface
      visible to a spectator from a given point; the apparent
      junction of the earth and sky.

            And when the morning sun shall raise his car Above
            the border of this horizon.           --Shak.

            All the horizon round Invested with bright rays.
                                                  --Milton.

   2. (Astron.)
      (a) A plane passing through the eye of the spectator and
          at right angles to the vertical at a given place; a
          plane tangent to the earth's surface at that place;
          called distinctively the sensible horizon.
      (b) A plane parallel to the sensible horizon of a place,
          and passing through the earth's center; -- called also
          {rational or celestial horizon}.
      (c) (Naut.) The unbroken line separating sky and water, as
          seen by an eye at a given elevation, no land being
          visible.

   3. (Geol.) The epoch or time during which a deposit was made.

            The strata all over the earth, which were formed at
            the same time, are said to belong to the same
            geological horizon.                   --Le Conte.

   4. (Painting) The chief horizontal line in a picture of any
      sort, which determines in the picture the height of the
      eye of the spectator; in an extended landscape, the
      representation of the natural horizon corresponds with
      this line.

   {Apparent horizon}. See under {Apparent}.

   {Artificial horizon}, a level mirror, as the surface of
      mercury in a shallow vessel, or a plane reflector adjusted
      to the true level artificially; -- used chiefly with the
      sextant for observing the double altitude of a celestial
      body.

   {Celestial horizon}. (Astron.) See def. 2, above.

   {Dip of the horizon} (Astron.), the vertical angle between
      the sensible horizon and a line to the visible horizon,
      the latter always being below the former.

   {Rational horizon}, and {Sensible horizon}. (Astron.) See
      def. 2, above.

   {Visible horizon}. See definitions 1 and 2, above.

Source : WordNet®

celestial horizon
     n : the great circle on the celestial sphere whose plane passes
         through the sensible horizon and the center of the Earth
         [syn: {horizon}]