Use the adjective geometric to describe anything that's decorated with simple shapes and lines. The geometric design of your new wallpaper, with its repeated triangles, makes your room look really sophisticated.
In math, geometric refers to geometry — any calculations involving the angles of a polygon or the diameter of a circle are geometric. The Greek root is geometria, "measurement of earth or land." The math definition of geometric is its original meaning, dating to the early 17th century. It wasn't until 1900 that the adjective was first used to describe a design style — specifically, a type of Greek pottery decoration characterized by lines and angles.