Not surprisingly, it is called a side!
A five-sided polygon is called a "pentagon."
It is called a Triacontagon
An 18 sided polygon is called a Octadecagon or a Octakaidecagon
A 6 sided polygon is called an hexagon
The name of a 12345 sided polygon is Myriatriatrictetrapentagon.
A fourteen-sided polygon is called a tetradecagon.
It is called a heptagon.
The 11 side polygon is called bysocleptron
For a nine side polygon its called a nonagon.For a nine side polygon its called a nonagon.For a nine side polygon its called a nonagon.
10 side polygon is called decogon
The name of a 10 side polygon is called a decagon