A 15-sided shape may be called a pentadecagon or pentakaidecagon, depending on who is doing the naming. More often, it is a polygon, which is an enclosed shape with any number of sides that do not cross. Polygons can be all convex, or they can be a combination of concave and convex vertices.
Formulas exist for creating names of polygons with a large number of sides. For instance, a polygon with 99 sides uses the prefix for 90, "enneacontakai," and the suffix for 9, "enneagon" to get "enneacontakaienneagon." Words for polygons with sides can go into the thousands. For instance, a polygon with 10,000 sides is a myriagon. Nonetheless, many times mathematicians will just use the simplified "34-gon" rather than the more complicated naming system."