The Collaborative International Dictionary of English v.0.48:

Rhombus \Rhom"bus\, n. [L.]
   Same as Rhomb, 1.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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