horizontal

American Heritage® Dictionary of the English Language, Fourth Edition

1. adj. Of, relating to, or near the horizon.
2. adj. Parallel to or in the plane of the horizon.
3. adj. At right angles to a vertical line.
4. adj. Occupying or restricted to the same level in a hierarchy: a horizontal study of verbal ability; a horizontal transfer by an employee.
5. n. Something, such as a line, plane, or object, that is horizontal.

Century Dictionary and Cyclopedia

1. Of or pertaining to the horizon.
2. 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. Specifically— In mech., acting or working, or placed, wholly or with respect to its main parts, in a level plane: as, a horizontal drill; a horizontal boring-machine; a horizontal saw-mill and mortising-machine; a horizontal pump; a horizontal escapement; a horizontal steam-engine.
3. Measured or contained in a plane of the horizon: as, horizontal distance.
4. 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.
5. n. In craniometry, the line drawn from the lower edge of the orbital cavity to the middle of the ear-cavity.
6. n. 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. It is named from its peculiar habit of growth. The main stem after reaching a certain height assumes a horizontal or drooping position, from which the branches ascend and repeat the action of the stem. The same is done by the secondary branches, all of which interlock and form an almost impenetrable thicket known as horizontal scrub.

Wiktionary

1. adj. perpendicular to the vertical; parallel to the plane of the horizon; level, flat.
2. adj. Pertaining to the horizon.
3. n. A horizontal component of a structure.

GNU Webster's 1913

1. adj. Pertaining to, or near, the horizon.
2. adj. Parallel to the horizon; on a level.
3. adj. Measured or contained in a plane of the horizon.

WordNet 3.0

1. adj. parallel to or in the plane of the horizon or a base line
2. n. something that is oriented horizontally