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:

Artificial \Ar`ti*fi"cial\, a. [L. artificialis, fr. artificium:
   cf. F. artificiel. See Artifice.]
   1. Made or contrived by art; produced or modified by human
      skill and labor, in opposition to natural; as, artificial
      heat or light, gems, salts, minerals, fountains, flowers.
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            Artificial strife
            Lives in these touches, livelier than life. --Shak.
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   2. Feigned; fictitious; assumed; affected; not genuine.
      "Artificial tears." --Shak.
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   3. Artful; cunning; crafty. [Obs.] --Shak.
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   4. Cultivated; not indigenous; not of spontaneous growth; as,
      artificial grasses. --Gibbon.
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   Artificial arguments (Rhet.), arguments invented by the
      speaker, in distinction from laws, authorities, and the
      like, which are called inartificial arguments or proofs.
      --Johnson.

   Artificial classification (Science), an arrangement based
      on superficial characters, and not expressing the true
      natural relations species; as, "the artificial system" in
      botany, which is the same as the Linn[ae]an system.

   Artificial horizon. See under Horizon.

   Artificial light, any light other than that which proceeds
      from the heavenly bodies.

   Artificial lines, lines on a sector or scale, so contrived
      as to represent the logarithmic sines and tangents, which,
      by the help of the line of numbers, solve, with tolerable
      exactness, questions in trigonometry, navigation, etc.

   Artificial numbers, logarithms.

   Artificial person (Law). See under Person.

   Artificial sines, tangents, etc., the same as logarithms
      of the natural sines, tangents, etc. --Hutton.
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WordNet (r) 3.0 (2006):

artificial horizon
    n 1: a navigational instrument based on a gyroscope; it
         artificially provides a simulated horizon for the pilot
         [syn: artificial horizon, gyro horizon, flight
         indicator]