An 18-sided polygon, sometimes also called an octakaidecagon.
Octadecagon
See also
Polygon, Regular Polygon, Trigonometry Angles--Pi/18Explore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Octadecagon." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Octadecagon.html