An icosahedron is a regular polyhedron with 20 faces, all of which are equilateral triangles. An icosahedron has 30 edges and 12 vertices. Five equilateral triangles meet at each vertex.
An icosahedron is an example of a Platonic solid, which is a polyhedron that is regular, convex, made of congruent faces of regular polygons, and has the same number of faces meeting at each vertex. Only five known shapes meet this criteria.
The other Platonic solids include the tetrahedron with four triangular faces, the cube, or hexahedron, with six square faces, the octahedron with eight triangular faces and the dodecahedron with twelve pentagonal faces.