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A regular nonagon is a nonagon which is both equilateral and equiangular.
That is, a regular polygon with $9$ sides.
That is, a nonagon in which all the sides are the same length, and all the vertices have the same angle:
Also known as
A regular nonagon can also be referred to as a regular enneagon.
The word nonagon is a hybrid word from the Latin nonus, meaning ninth, and gon, deriving from the Greek for corner.
The arguably more correct (but less common) word enneagon is purely Greek in derivation, the first part coming from ennea, meaning nine.