Par·al·lel·o·gram n.  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, 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.

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 Pol·y·gon n.  Geom. A plane figure having many angles, and consequently many sides; esp., one whose perimeter consists of more than four sides; any figure having many angles.
 Polygon of forces Mech., a polygonal figure, the sides of which, taken successively, represent, in length and direction, several forces acting simultaneously upon one point, so that the side necessary to complete the figure represents the resultant of those forces.  Cf. Parallelogram of forces, under Parallelogram.

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