Definition of: horizontal

(hôr′ə·zon′təl, hor′-) adjective
1. Parallel to the horizon; on a level.
2. Included or measured in a plane of the horizon.
3. Of, pertaining to, on, or close to the horizon.
4. Equal and uniform: a horizontal tariff.
5. Made up of similar units: a horizontal trust.
—noun A line or plane assumed, for the purpose of measurement or description, to be parallel with the horizon.
—hor′i·zon′tal·ly adverb Synonyms (adj.): flat, level, plain, plane. For practical purposes level and horizontal are identical, but level is more loosely used of that which has no especially noticeable elevations or inequalities; as, a level road. Flat applies to a surface only, and, in the first, most usual sense, to a surface that is horizontal or level in all directions; flat is also applied to any plane surface without irregularities or elevations, as a picture may be painted on the flat surface of a perpendicular wall. Plane applies only to a surface, and is used with more mathematical exactness than flat. Plain, originally the same word as plane, is now rarely used except in senses pertaining to level ground. We speak of a horizontal line, a flat morass, a level road, a plain country, a plane surface (especially in the scientific sense). See FLAT, LEVEL. Antonyms: broken, hilly, inclined, irregular, rolling, rough, rugged, slanting, sloping, uneven.