The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   Parallelogram of velocities, parallelogram of forces,
   parallelogram of accelerations, parallelogram of 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.
      [1913 Webster]