It's either called a 'hendecagon' or an 'undecagon'. (It has two names, I'm not implying my uncertainty of the answer).
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A 9 sided polygon is a Nonagon.
Some examples are: 5 sided polygon is a pentagon, 6 sided polygon is an hexagon and an 8 sided polygon is an octagon
The name of a 18 sided polygon is Octadecagon.
A hexagon is a six-sided polygon. A four-sided polygon is a quadrilateral, quadrangle, or tetragon.