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An icosahedron is a polyhedron which has $20$ faces.

Regular Icosahedron

A regular icosahedron is an icosahedron whose $20$ faces are all congruent equilateral triangles.

Truncated Icosahedron

A truncated icosahedron is an icosahedron whose $12$ vertices are truncated such that one third of each edge is cut off at each of both ends.

Also see

  • Results about icosahedra can be found here.

Linguistic Note

The word icosahedron derives from the Classical Greek εἰκοσάεδρον:

eíkosi (εἴκοσι), meaning twenty
hedron (a form of ἕδρα), meaning base or seat.

The technically correct plural of icosahedron is icosahedra, but the word icosahedrons can often be found.