The Collaborative International Dictionary of English v.0.48:

Decahedron \Dec`a*he"dron\ (d[e^]k`[.a]*h[=e]"dr[o^]n), n.; pl.
   E. Decahedrons (d[e^]k`[.a]*h[=e]"dr[o^]nz), L.
   Decahedra. [Pref. deca- + Gr. 'e`dra a seat, a base, fr.
   'e`zesqai to sit: cf. F. d['e]ca[`e]dre.] (Geom.)
   A solid figure or body inclosed by ten plane surfaces.
   [Written also, less correctly, decaedron.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

decahedron
    n 1: any polyhedron having ten plane faces