The Collaborative International Dictionary of English v.0.48:

Rhombohedral \Rhom`bo*he"dral\, a. (Geom. & Crystallog.)
   Related to the rhombohedron; presenting the form of a
   rhombohedron, or a form derivable from a rhombohedron;
   relating to a system of forms including the rhombohedron and
   scalenohedron.
   [1913 Webster]

   Rhombohedral iron ore (Min.) See Hematite.

   Rhombohedral system (Crystallog.), a division of the
      hexagonal system embracing the rhombohedron,
      scalenohedron, etc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

rhombohedral
    adj 1: having threefold symmetry [syn: rhombohedral,
           trigonal]