A polyhedron with 20 faces. On a regular icosahedron each face is an equilateral triangles.

• 20 triangular faces.
• 30 edges.
• 12 vertices.

5 faces (and edges) meet at every vertex.

So the dual polyhedron is the dodecahedron.

A regular icosahedron is known to the gaming world as a d20.

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 triangles. Five triangles meet to form each solid angle of the polyhedron.

© Webster 1913.