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.
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            And when the morning sun shall raise his car
            Above the border of this horizon.     --Shak.
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            All the horizon round
            Invested with bright rays.            --Milton.
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   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.
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   3. (Geol.) The epoch or time during which a deposit was made.
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            The strata all over the earth, which were formed at
            the same time, are said to belong to the same
            geological horizon.                   --Le Conte.
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   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.
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   5. The limit of a person's range of perception, capabilities,
      or experience; as, children raised in the inner city have
      limited horizons.
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   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.
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   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.
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The Collaborative International Dictionary of English v.0.48:

Apparent \Ap*par"ent\, a. [F. apparent, L. apparens, -entis, p.
   pr. of apparere. See Appear.]
   1. Capable of being seen, or easily seen; open to view;
      visible to the eye; within sight or view.
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            The moon . . . apparent queen.        --Milton.
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   2. Clear or manifest to the understanding; plain; evident;
      obvious; known; palpable; indubitable.
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            It is apparent foul play.             --Shak.
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   3. Appearing to the eye or mind (distinguished from, but not
      necessarily opposed to, true or real); seeming; as the
      apparent motion or diameter of the sun.
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            To live on terms of civility, and even of apparent
            friendship.                           --Macaulay.
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            What Berkeley calls visible magnitude was by
            astronomers called apparent magnitude. --Reid.
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   Apparent horizon, the circle which in a level plain bounds
      our view, and is formed by the apparent meeting of the
      earth and heavens, as distinguished from the rational
      horizon.

   Apparent time. See Time.

   Heir apparent (Law), one whose to an estate is indefeasible
      if he survives the ancestor; -- in distinction from
      presumptive heir. See Presumptive.
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   Syn: Visible; distinct; plain; obvious; clear; certain;
        evident; manifest; indubitable; notorious.
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WordNet (r) 3.0 (2006):

apparent horizon
    n 1: the line at which the sky and Earth appear to meet [syn:
         horizon, apparent horizon, visible horizon, sensible
         horizon, skyline]