What is a closed figure made up of line segments?

A closed figure made up of line segments is called a "polygon." The term "polygon" is derived from the Greek words "poly," which means "many," and "gon," which means "angle."

A polygon is a two-dimensional geometric structure containing straight lines called "sides." One side of a polygon is connected to two other sides at their endpoints. If a polygon's sides and angles are equilateral and equiangular, respectively, then it is called a "regular polygon." Otherwise, the polygon is irregular. Other types of polygons include concave or convex and simple or complex. Some common polygons include triangles, pentagons, nonagons, quadrilaterals, octagons, decagons, pentagons, hexagons and heptagons.