What does "perpendicular" mean?

Perpendicular: (Geom.) At right angles to a given line or surface; as, the line ad is perpendicular to the line bc. Perpendicular style (Arch.), a name given to the latest variety of English Gothic architecture, which prevailed from the close of the 14th century to the early part of the 16th; -- probably so called from the vertical style of its window mullions.

Additional senses

  1. 2.A line at right angles to the plane of the horizon; a vertical line or direction.
  2. 3.(Geom.) A line or plane falling at right angles on another line or surface, or making equal angles with it on each side.

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