What is an 11-sided polygon called?

An 11-sided polygon is called a hendecagon. It is also known as the undecagon or unidecagon. However, the term "hendecagon" takes precedence over these variants because of its consistency in using a Greek prefix and suffix, instead of combining a Roman prefix with a Greek suffix.

The name hendecagon comes from Greek "hendeka," which means eleven, and "gon," which means corner. The basic 11-sided polygon has Schläfli symbol, a notation of the form {p,q,r,...} that defines regular polygons, polyhedra, and their higher-dimensional counterparts; it cannot be constructed using the classical Greek rules of geometric construction. The rim of the U.S. Susan B. Anthony dollar coin is a hendecagon.