What does "rhomb" mean?

Rhomb: (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.