The Collaborative International Dictionary of English v.0.48:

Horizontal \Hor`i*zon"tal\, a. [Cf. F. horizontal.]
   1. Pertaining to, or near, the horizon. "Horizontal misty
      air." --Milton.
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   2. Parallel to the horizon; on a level; as, a horizontalline
      or surface.
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   3. Measured or contained in a plane of the horizon; as,
      horizontal distance.
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   Horizontal drill, a drilling machine having a horizontal
      drill spindle.

   Horizontal engine, one the piston of which works
      horizontally.

   Horizontal fire (Mil.), the fire of ordnance and small arms
      at point-blank range or at low angles of elevation.

   Horizontal force (Physics), the horizontal component of the
      earth's magnetic force.

   Horizontal line (Descriptive Geometry & Drawing), a
      constructive line, either drawn or imagined, which passes
      through the point of sight, and is the chief line in the
      projection upon which all verticals are fixed, and upon
      which all vanishing points are found.

   Horizontal parallax. See under Parallax.

   Horizontal plane (Descriptive Geometry), a plane parallel
      to the horizon, upon which it is assumed that objects are
      projected. See Projection. It is upon the horizontal
      plane that the ground plan of the buildings is supposed to
      be drawn.

   Horizontal projection, a projection made on a plane
      parallel to the horizon.

   Horizontal range (Gunnery), the distance in a horizontal
      plane to which a gun will throw a projectile.

   Horizontal water wheel, a water wheel in which the axis is
      vertical, the buckets or floats revolving in a horizontal
      plane, as in most turbines.
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