1. a quadrilateral whose opposite sides are both parallel and equal in length;

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]WordNet (r) 3.0 (2006):

parallelogram n 1: a quadrilateral whose opposite sides are both parallel and equal in length [ant: trapezium]