What is a 100-sided shape called?

A 100-sided polygon is a hectagon. It comes from the Greek prefix "hecta," which means "100," and "gon" from "gonu," which means "knee," but later denoted "angle."

A polygon is a closed two-dimensional geometric figure containing several line segments, where each side intersects only two other segments at what is called a vertex. Some commonly used prefixes that denote the number of sides of a polygon include "tetra," "penta," "hexa," "hepta" and "octa," used for the numbers 4, 5, 6, 7 and 8, respectively. Generally accepted polygonal names are pentagon, hexagon, heptagon, octagon, nonagon and decagon. An "n-gon" is a type of polygon with n number of sides, where n is a variable.