The Collaborative International Dictionary of English v.0.48:

Icosahedron \I`co*sa*he"dron\, n. [Gr. ?; ? twenty + ? seat,
   base, fr. ? to sit.] (Geom.)
   A solid bounded by twenty sides or faces.
   [1913 Webster]

   Regular icosahedron, one of the five regular polyhedrons,
      bounded by twenty equilateral triangules. Five triangles
      meet to form each solid angle of the polyhedron.
      [1913 Webster]

WordNet (r) 3.0 (2006):

icosahedron
    n 1: any polyhedron having twenty plane faces