Synonyms for Horizontal:

adj.

flat (adjective)
planar, even, flush, flat, tabular, prairie-like, procumbent, level.
horizontal (adjective)
flat, even, level, planar.
lying flat (adjective)
parallel, uniform, recumbent, even, flush, straight, smooth, regular, level.
recumbent (adjective)
recumbent.

n.

curvature, outline, diagonal, arc, axis, line, inclined plane, diameter, chord. even (noun)
regular, uniform, smooth.
horizontal (noun)
level, flat, crosswise, swimming, naiant.
level (noun)
straight, parallel.

Other synonyms:

inclined plane, diameter, decumbent, smooth, procumbent. prostrate. parallel. square. line. straight.

Usage examples for horizontal

  1. Roofs were timbered with horizontal beams. – Our Legal Heritage, 4th Ed. by S. A. Reilly
  2. The horizontal beam seems too long; out of proportion. – Prison Memoirs of an Anarchist by Alexander Berkman
  3. There was a small outer tent, which was little more than a strip of canvas thrown over a horizontal pole and shielding a rough fireplace for rainy weather, and within was the little dwelling- tent, with a cot, and even a tiny table. – The Boy With the U. S. Foresters by Francis Rolt-Wheeler