The Collaborative International Dictionary of English v.0.48:

Perpendicular \Per`pen*dic"u*lar\, a. [L. perpendicularis,
   perpendicularius: cf. F. perpendiculaire. See Perpendicle,
   Pension.]
   1. 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.
      [1913 Webster]

   2. (Geom.) At right angles to a given line or surface; as,
      the line ad is perpendicular to the line bc.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

perpendicular style
    n 1: a Gothic style in 14th and 15th century England;
         characterized by vertical lines and a four-centered (Tudor)
         arch and fan vaulting [syn: perpendicular, perpendicular
         style, English-Gothic, English-Gothic architecture]