Definition of icosahedron:

part of speech: noun

A solid of twenty equal sides.

Usage examples:

• Within the Earth's orbit inscribe an icosahedron the sphere inscribed in it will be Venus's orbit. Thomas Orchard in "The Astronomy of Milton's 'Paradise Lost'".

• If you cut out a piece of cardboard of the form shown in the smaller diagram, and cut half through along the dotted lines, it will fold up and form a perfect icosahedron Henry Ernest Dudeney in "Amusements in Mathematics".

• The five regular solids, in appropriate order- Octahedron, Icosahedron Dodecahedron, Tetrahedron, Cube. Oliver Lodge in "Pioneers of Science".

• Now, inscribe in the earth an icosahedron the sphere inscribed in it will be Venus: inscribe an octahedron in Venus: the circle inscribed in it will be Mercury. Walter W. Bryant in "Kepler".