What are the characteristics of a polygon

Equal side lengths and equal angles between sides

unless it is an irregular polygon then it has unequal side lengths and unequal angles.

A regular polygon has equal length sides with equal angles between each side. Any other polygon is an irregular polygon, which by definition has unequal length sides and unequal angles between sides. Circles and shapes that include curves are not polygons - a polygon, by definition, is made up of straight lines.