\Rhomb\, n. [L. rhombus, Gr. ??? rhomb, a spinning top,
magic wheel, fr. ??? to turn or whirl round, perhaps akin to
E. wrench: cf. F. rhombe. Cf. {Rhombus}, {Rhumb}.]
1. (Geom.) An equilateral parallelogram, or quadrilateral
   figure whose sides are equal and the opposite sides
   parallel. The angles may be unequal, two being obtuse and
   two acute, as in the cut, or the angles may be equal, in
   which case it is usually called a square.
2. (Geom.) A rhombohedron.
{Fresnel's rhomb} (Opt.), a rhomb or oblique parallelopiped
   of crown or St. Gobain glass so cut that a ray of light
   entering one of its faces at right angles shall emerge at
   right angles at the opposite face, after undergoing within
   the rhomb, at other faces, two reflections. It is used to
   produce a ray circularly polarized from a plane-polarized
   ray, or the reverse. --Nichol.