A figure that is not a polygon?

A polygon is a plane closed shape that is bounded by straight lines. A figure that is not a polygon is one the does not satisfy all three requirements.

If the boundaries of the shape move in three (or more) dimensions then it cannot be a polygon. For example, consider a die, ABCDEFGH where the square EFGH is directly under ABCD. Then ABCGHDA is a closed figure, bounded by straight lines but it is not a plane figure.

If ABCD is a square, then ABC is not a polygon because it is not closed.

A circle, ellipse, semicircle, a squiggle are not polygons because they are bounded by sides that are no all straight.