horizontal

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Of, relating to, or near the horizon.
• adjective Parallel to or in the plane of the horizon.
• adjective At right angles to a vertical line.
• adjective Occupying or restricted to the same level in a hierarchy.
• adjective Medicine Of or relating to transmission of infection by contact with or physical proximity to an infected individual or fomite.
• noun Something, such as a line, plane, or object, that is horizontal.

from The Century Dictionary.

• noun Anodopetalum biglandulosum, a Tasmanian plant of the family Cunoniaceæ, which sometimes forms a tree 60–70 feet high, with a trunk 4 feet in circumference.
• Of or pertaining to the horizon.
• Parallel to the horizon; at right angles to the direction of gravity at any station; being on a level; not vertical nor inclined: as, a horizontal line or surface; a horizontal position.
• Measured or contained in a plane of the horizon: as, horizontal distance.
• In figured bass, a dash under a note indicating that the tones of the last figured chord are to be continued without regard to the tone of the bass.
• noun In craniometry, the line drawn from the lower edge of the orbital cavity to the middle of the ear-cavity.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Pertaining to, or near, the horizon.
• adjective Parallel to the horizon; on a level.
• adjective Measured or contained in a plane of the horizon.
• adjective a drilling machine having a horizontal drill spindle.
• adjective one the piston of which works horizontally.
• adjective (Mil.) the fire of ordnance and small arms at point-blank range or at low angles of elevation.
• adjective (Physics) the horizontal component of the earth's magnetic force.
• adjective (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.
• adjective See under Parallax.
• adjective (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.
• adjective a projection made on a plane parallel to the horizon.
• adjective (Gunnery) the distance in a horizontal plane to which a gun will throw a projectile.
• adjective a water wheel in which the axis is vertical, the buckets or floats revolving in a horizontal plane, as in most turbines.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective perpendicular to the vertical; parallel to the plane of the horizon; level, flat.
• adjective archaic Pertaining to the horizon.
• noun A horizontal component of a structure.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective parallel to or in the plane of the horizon or a base line
• noun something that is oriented horizontally