Yo can say 13gon or 14gon because really there is no name for it.
The name of a 14 sided polygon is tetradecagon.
A polygon with 14 diagonals will have 7 sides
The name for this polygon is a nonagon,
The name of a 14 sided polygon is Tetradecagon.
The name of a 7 sided polygon is Heptagon.
A hendecagon or an undecagon is the name of a polygon which has eleven sides.
Apothem!
Not surprisingly, it is called a side!
"Heptagon" is the name of a 7-sided polygon.
If measured on the inside of the polygon, the it is an interior angle.
A Decatetragon