Regular 20 sided solid is an icosahedron, so a plane equivalent is presumably an icosagon?
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An icosagon
A polygon with 20 sides is called either a 20-gon or an icosagon.
A 20-sided polyhedron is an icosahedron. A 20-sided polygon is an icosagon.
Icosahedron is a 20 sided polygon.
a 4 sided shape is called a quadrilateral