Which term describes a shape that curves inward?

The term that describes a shape that curves inward is "concave." When we refer to a concave shape, we are identifying a figure that has at least one indentation or inward curve. This is in contrast to shapes that are convex, which bulge outward and have every line segment connecting any two points on the shape lying entirely within or on the boundary of the shape.

In geometry, concave shapes can be visualized as having a "cave" or an inward dip, making it quite distinct from other types of shapes. For instance, if a shape were to resemble the interior of a bowl or a portion of a circle where the "opening" is on the outside, it would be classified as concave.

The other terms presented have different geometric meanings. A flat shape refers to a two-dimensional figure with no curvature, acute describes angles that are less than 90 degrees, and convex shapes are characterized by their outward bulges. Therefore, identifying "concave" as the term for inward-curving shapes reflects a proper understanding of geometric terminology.