Which term describes a shape that is curved inward?

The term that describes a shape that is curved inward is "concave." When a surface or outline dips inward, it creates a bowl-like effect. This term is often used in various contexts, such as geometry and optics, to describe lenses and shapes. For example, in the case of lenses, a concave lens is one that curves inward, which causes light rays to diverge.

In contrast, a convex shape curves outward, resembling the exterior of a sphere, while flat refers to a level or plane surface without any curvature. Elongated depicts an extended shape, which does not apply to the concept of inward curvature. By understanding these definitions, it becomes clear why "concave" is the appropriate term to characterize a shape that is curved inward.