An icosahedron is a polyhedron with 20 faces. A regular icosahedron
has triangular faces each of which is an equilateral triangle.
The word icos comes from the Greek for twenty.
In geometry, an icosahedron is any polyhedron having 20 faces,
but usually a regular icosahedron is implied, which has equilateral
triangles as faces. The regular icosahedron is one of the
five Platonic solids. It is a convex regular polyhedron composed
of twenty triangular faces, with five meeting at each of the
twelve vertices. It has 30 edges and 12 vertices. Its dual
polyhedron is the dodecahedron.
Example of an Acute Angled Triangle: