hor·i·zon·tal /ˌhɔrəˈzɑntḷ, ˌhɑr-/
1. Pertaining to, or near, the horizon. “Horizontal misty air.”
2. Parallel to the horizon; on a level; as, a horizontalline or surface.
3. Measured or contained in a plane of the horizon; as, horizontal distance.
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.
adj : parallel to or in the plane of the horizon or a base line;
"a horizontal surface" [ant: vertical, inclined]
n : something that is oriented horizontally