An eleven sided polygon (2D shape with straight sides) is called a Hendecagon.
I often set this as a research puzzle for my mathematics classes!
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A hendecagon or an undecagon is the name of a polygon which has eleven sides.
The hendecagon is a polygon with eleven sides.
It is called Enneagon for eleven sides.
A polygon with eleven sides and corners is called a hendecagon. Occasionally, it may be referred to as an undecagon.
Hexadecagon (16 sides) and Octadecagon (18 sides) both fit. Am I missing something?