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Definition of perpendicular:
 A line at right angles to the plane of the horizon; a vertical line or direction.
 A line or plane falling at right angles on another line or surface, or making equal angles with it on each side.
 At right angles to a given line or surface; as, the line ad is perpendicular to the line bc.
 Exactly upright or vertical; pointing to the zenith; at right angles to the plane of the horizon; extending in a right line from any point toward the center of the earth.
Synonyms:
unsloped, EnglishGothic architecture, carbuncle, inclined plane, comfortable, plumb, level, appointed, englishgothic, raised, standup, minute, baronial, chord, upright, right, beachfront, brownstone, flat, upstanding, perpendicular style, carpeted, diameter, axis, plumb line, median, degree, vertical, oblique, outline, square, steep, cavernous, line, rectangular, arc, obtuse, normal, cotangent, curvature, colonial, burnedout, standing, diagonal, cosine, upended, cos, orthogonal, horizontal.
 straight (part of speech: adjective)

vertical (part of speech: adjective)
 upanddown,
 sheer,
 upright,
 vertical,
 longitudinal,
 steep,
 erect,
 at degrees
Usage examples:

Down the rugged and almost perpendicular descent, the dog, without any hesitation, began to make his way.
 "True Stories about Cats and Dogs", Eliza Lee Follen. 
On the other side it was perpendicular.
 "The Scottish Chiefs", Jane Porter. 
Spaniards were not birds of prey to fly up those perpendicular heights, and for beings without wings the thing was impossible.
 "Project Gutenberg History of The Netherlands, 15551623, Complete", John Lothrop Motley.