Which word describes a surface that curves inward?

The term that describes a surface which curves inward is "concave." This word specifically refers to a shape that is hollowed or depressed, creating a bowl-like appearance. When visualizing a concave surface, think of a cave or a bowl, where the inner part is recessed compared to the outer edges, which can be illustrated by something like a spoon's interior.

In contrast, a convex surface curves outward, like the exterior of a sphere. A flat surface has no curvature at all, remaining level and even. Angular refers to shapes with distinct edges and corners, which does not accurately represent the curvature in question. Thus, "concave" is the most precise and fitting term when describing a surface that curves inward.