Remmeber that an octagon is a polygon formed by chain of 8 straight line segments.
The first valid name for a polygon of eight sides is octagon (from greek oktagonon, which means eight angles)
The second valid name for a polygon of eight sides is eight-sided polygon.
The third valid name of a polygon of eight sides is 8-gon. After 6 sides, mathematicians usually refer to these polygons as n-gons, so an eight-sided polygon will be an 8-gon.
We can conclude that the three valid names for an octagon are: octagon, eight-sided polygon, and 8-gon.