ICOSAHEDRON (Gr. eiro<n, twenty, and '(Spa, a face or base), in geometry, a solid enclosed by twenty faces. The " regular icosahedron " is one of the Platonic solids; the " great icosahedron " is a Kepler-Poinsot solid; and the " truncated icosahedron " is an Archimedean solid (see POLYHEDRON). In crystallography the icosahedron is a possible form, but it has not been observed; it is closely simulated by a combination of the octahedron and pentagonal dodecahedron, which has twenty triangular faces, but only eight are equilateral, the remaining twelve being isosceles (see CRYSTALLOGRAPHY).
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)