An icosahedron is in fact not a polygon at all, but a polyhedron. As a polyhedron it is regular however.
The regular icosahedron is one of only five possible regular polyhedra. It has 20 faces, each of which is an equilateral triangle.
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There are eight different types of semiregular tessellations. Also called Archimedean tessellations, they occur when two or more convex regular polygons form tessellations of the plane in a way each polygon vertex is surrounded by the same polygons and in the same order.
A icosahedron.
Icosahedron is a 20 sided polygon.
Regular 20 sided solid is an icosahedron, so a plane equivalent is presumably an icosagon?
See the answer below.