An icosahedron is any polyhedron with twenty faces. In fact, the term is almost always used to refer specifically to a polyhedron with twenty triangular faces, and modifying words or alternate terminology are used to refer to other twenty-sided polyhedra, as in the case of the rhombic icosahedron.

The regular icosahedron is one of the five Platonic solids: its faces are all equilateral triangles. It has twenty vertices and thirty edges. Five faces meet at each vertex. It is dual to the regular dodecahedron.

A soccer ball is an example of an icosahedron with the vertices flattened into pentagonal faces.

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