The Collaborative International Dictionary of English v.0.48:

Horizon \Ho*ri"zon\, n. [F., fr. L. horizon, fr. Gr. ? (sc. ?)
   the bounding line, horizon, fr. ? to bound, fr. ? boundary,
   limit.]
   1. The line 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.
      [1913 Webster]

            And when the morning sun shall raise his car
            Above the border of this horizon.     --Shak.
      [1913 Webster]

            All the horizon round
            Invested with bright rays.            --Milton.
      [1913 Webster]

   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 horizon 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.
          [1913 Webster]

   3. (Geol.) The epoch or time during which a deposit was made.
      [1913 Webster]

            The strata all over the earth, which were formed at
            the same time, are said to belong to the same
            geological horizon.                   --Le Conte.
      [1913 Webster]

   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.
      [1913 Webster]

   5. The limit of a person's range of perception, capabilities,
      or experience; as, children raised in the inner city have
      limited horizons.
      [PJC]

   6. [fig.] A boundary point or line, or a time point, beyond
      which new knowledge or experiences may be found; as, more
      powerful computers are just over the horizon.
      [PJC]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

horizon
    n 1: the line at which the sky and Earth appear to meet [syn:
         horizon, apparent horizon, visible horizon, sensible
         horizon, skyline]
    2: the range of interest or activity that can be anticipated;
       "It is beyond the horizon of present knowledge" [syn:
       horizon, view, purview]
    3: a specific layer or stratum of soil or subsoil in a vertical
       cross section of land
    4: the great circle on the celestial sphere whose plane passes
       through the sensible horizon and the center of the Earth
       [syn: horizon, celestial horizon]

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "horizon":
   azimuth, background, clear sight, color vision, command, compass,
   cone vision, day vision, daylight vision, discernment, domination,
   eye, eye-mindedness, eyereach, eyeshot, eyesight, false horizon,
   farsight, farsightedness, field of view, field of vision,
   keen sight, ken, limit of vision, limits, line of sight, naked eye,
   night vision, offing, outlook, outlook over, perception,
   peripheral field, peripheral vision, perspective, perspicacity,
   perspicuity, photopia, power of sight, prospect, purview,
   quick sight, range, reach, remote distance, rod vision, scan,
   scope, scope of vision, scotopia, sea line, seeing, sense of sight,
   sensible horizon, sight, sightedness, sightliness, skyline, survey,
   sweep, the distance, twilight vision, unobstructed vision,
   vanishing point, view, visible horizon, vision, vista,
   visual acuity, visual field, visual sense