Source : Webster's Revised Unabridged Dictionary (1913)

Parallelogram \Par`al*lel"o*gram\, n. [Gr. ?; ? parallel + ? to
   write: cf. F. parall['e]logramme. See {Parallel}, and
   {-gram}.] (Geom.)
   A right-lined quadrilateral figure, whose opposite sides are
   parallel, and consequently equal; -- sometimes restricted in
   popular usage to a rectangle, or quadrilateral figure which
   is longer than it is broad, and with right angles.

   {Parallelogram of velocities}, {forces}, {accelerations},
   {momenta}, etc. (Mech.), a parallelogram the diagonal of
      which represents the resultant of two velocities, forces,
      accelerations, momenta, etc., both in quantity and
      direction, when the velocities, forces, accelerations,
      momenta, etc., are represented in quantity and direction
      by the two adjacent sides of the parallelogram.

Source : WordNet®

parallelogram
     n : a quadrilateral whose opposite sides are both parallel and
         equal in length [ant: {trapezium}]