The Collaborative International Dictionary of English v.0.48:

octahedron \oc`ta*he"dron\ ([o^]k`t[.a]*h[=e]"dr[o^]n), n. [Gr.
   'okta`edron, fr. 'okta`edros eight-sided; 'okta- (for 'oktw`
   eight) + "e`dra seat, base, from "e`zesqai to sit.] (Geom.)
   A solid bounded by eight faces. The regular octahedron is
   contained by eight equal equilateral triangles.
   [1913 Webster]

WordNet (r) 3.0 (2006):

octahedron
    n 1: any polyhedron having eight plane faces