The Collaborative International Dictionary of English v.0.48:

Rectangle \Rec"tan`gle\ (r?k"t??`g'l), n. [F., fr. L. rectus
   right + angulus angle. See Right, and Angle.] (Geom.)
   A four-sided figure having only right angles; a right-angled
   parallelogram.
   [1913 Webster]

   Note: As the area of a rectangle is expressed by the product
         of its two dimensions, the term rectangle is sometimes
         used for product; as, the rectangle of a and b, that
         is, ab.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Rectangle \Rec"tan`gle\, a.
   Rectangular. [R.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

rectangle
    n 1: a parallelogram with four right angles

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "rectangle":
   Little Joe, biquadrate, four, four-part diaphony, foursome, quad,
   quadrangle, quadrature, quadrennium, quadrilateral, quadrille,
   quadrinomial, quadruplet, quartet, quatern, quaternary, quaternion,
   quaternity, quatrefoil, square, square dance, squaring, tetrad,
   tetragon, tetragram, tetragrammaton, tetrahedron, tetralogy,
   tetraphony, tetrapody