Source : Webster's Revised Unabridged Dictionary (1913)

Rhombohedron \Rhom`bo*he"dron\, n. [NL., fr. Gr. ??? rhomb + ???
   seat, base.] (Geom. & Crystallog.)
   A solid contained by six rhomboids; a parallelopiped.

Source : WordNet®

rhombohedron
     n : a parallelepiped bounded by six similar faces (either
         rhombuses or parallelograms)