A nine-sided plane figure is called a nonagon or an ennneagon.

en.wikipedia.org/wiki/Nonagon

In geometry, a nonagon /ˈnɒnəɡɒn is a nine-sided polygon or 9-gon. The name
"nonagon" is a prefix hybrid formation, from Latin (nonus, "ninth" + gonon), ...

www.reference.com/math/nine-sided-shape-called-680afddabbcb9b5c

A nonagon is a closed shape that has nine sides. If it is a regular nonagon, all
nine sides are the same length, and all the angles are equal....

www.math.com/tables/geometry/polygons.htm

A closed plane figure made up of several line segments that are joined ... Two or
more of these points are called vertices. ... +5 ...pentagon. +6 ...hexagon. +7 ...
heptagon. +8 ...octagon. +9 ...enneagon ... 28 sided polygon - Icosikaioctagon.

mathcentral.uregina.ca/qq/database/qq.09.96/rosa1.html

What do you call a 13 sided polygon? Is there a ... There seems to be an "official"
set of names for polygons, uniformly derived from the Greek (after 4 sides). ... 9 ...

www.algebra.com/algebra/homework/Polygons/Polygons.faq.question.18713.html

You can put this solution on YOUR website! A polygon is the name given to an n
sided figure where n can take any value starting from 3. Remember Penta ...

mathforum.org/library/drmath/view/59240.html

What is the name of a 7-sided figure? a 9-sided figure? Is one of these ever
called a septagon? If.

www.mcwdn.org/Geometry/OtherPolygons.html

Other Polygons. When you have finished this page, try the Other Polygons Quiz.
... Pentagons - Five sided polygons. Hexagon ... Nonagon - Nine sided polygon.

faculty.kutztown.edu/schaeffe/tutorials/general/polygons.html

9. = nona or ennea. 10. = deca. 11. = undeca or hendeca. 12. = dodeca. 13. =
triskaideca. 14 ... Example: A 35-sided polygon is called a "triacontakaipentagon.
" ...

www.mathsisfun.com/shape.html

... straight sides. To be a regular polygon all the sides and angles must be the
same: ... Nonagon - 9 Sides, 2d decagon ... Any 4 sided 2D shape, 2d rectangle