The Icosahedron
The regular icosahedron is one of the five Platonic solids. It is bounded by 20 equilateral triangles, and has 12 vertices and 30 edges. Its dual is the dodecahedron.
It can be cut into two pentagonal pyramids and a pentagonal antiprism, or a pentagonal pyramid and a gyroelongated pentagonal pyramid (J11).
Projections
In order to be able to identify the icosahedron in various projections of 4D objects, it is useful to know how it appears from various viewpoints. The following are some of the viewpoints that are commonly encountered:
Projection  Envelope  Description 

Regular decagon  Vertexfirst parallel projection. 

Hexagon  Edgefirst parallel projection. Four of the icosahedron's faces project onto the top two and bottom two edges of the projection envelope. 

Regular hexagon  Facefirst parallel projection. 

Nonuniform octagon  Parallel projection, parallel to two opposite faces. The two opposite faces project to the top and bottom edges of the projection envelope. 
Coordinates
The Cartesian coordinates of an origincentered regular icosahedron of edge length 2 are:
 (0, ±1, ±φ)
 (±1, ±φ, 0)
 (±φ, 0, ±1)
where φ=(1+√5)/2 is the Golden Ratio.
Occurrences
The icosahedron occurs as cells in the following 4D uniform polytopes:
 The snub 24cell;
 The truncated 600cell;
 The rectified 600cell.
In addition, it also occurs in many CRF polychora, including (but not limited to):