What is the name of a 51-sided polygon?

A 51-sided polygon is called a "pentacontakaihenagon." In this term, "penta-" refers to five, "conta-" refers to twenty, and "kai" means "and," while "henagon" indicates a polygon. Therefore, it literally translates to a polygon with fifty-one sides. However, it is often simply referred to as a 51-gon in casual contexts.