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: Webster's Revised Unabridged Dictionary (1913)