A polygon with 1 billion sides is called a gigagon.
While the term "gigagon" isn't as commonly used as names for polygons with fewer sides (like triangle, square, pentagon, etc.), it's the logical extension of the naming convention based on Greek numerical prefixes. The prefix "giga-" denotes a billion (109). Therefore, a "gigagon" aptly describes a polygon with one billion sides.
It's worth noting that as the number of sides in a polygon increases dramatically, its shape visually approximates a circle. A gigagon would be, for all practical purposes, indistinguishable from a circle to the naked eye.
