Hendecagon

A hendecagon is an 11-sided polygon.
Undecagon is incorrect — the prefix is properly Greek, not Latin.
A regular hendecagon has internal angles of 147.272727... degrees.
The area of a regular hendecagon with side length a is given by
:
A regular hendecagon is not constructible with straightedge and compass.

The Canadian dollar coin, the Loonie, is a hendecagon. The interior design of the (US coin) Susan B. Anthony silver dollar is also a hendecagon.

See also