Source : Webster's Revised Unabridged Dictionary (1913)

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

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

Source : WordNet®

regular icosahedron
     n : an icosahedron with twenty equilateral triangles as faces