The Collaborative International Dictionary of English v.0.48:

Rhomb \Rhomb\ (r[o^]mb or r[o^]m; 277), n. [L. rhombus, Gr.
   "ro`mbos rhomb, a spinning top, magic wheel, fr. "re`mbein 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.
      [1913 Webster]

   2. (Geom.) A rhombohedron.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

rhomb
    n 1: a parallelogram with four equal sides; an oblique-angled
         equilateral parallelogram [syn: rhombus, rhomb,
         diamond]