An arch is a shape that resembles an upside down "U." You may find this shape in a carefully tweezed eyebrow or in the famous golden pair that make you hunger for a Big Mac.
In architecture, an arch is an opening that is often found supporting the weight of a something above it — like a bridge or a wall. In downtown St. Louis, you can visit the Gateway Arch — a freestanding catenary arch that is 630 feet tall and 630 feet wide. As a verb, arch means to make an arch-like shape. "She stretched her back by arching it into a backbend." As an adjective, arch can describe something mischievous or sly: "He teased his friend with an arch comment about his shyness around girls."