concave

This term describes a hollow or sunken geometry, evoking the image of a cave or a bowl. It is primarily used in technical, scientific, or architectural contexts to describe how a surface reflects light or holds volume. It carries a neutral, descriptive tone rather than an emotional one. In optics, it is strictly contrasted with convex. While a convex surface bulges outward, a concave surface dips inward, creating a focal point that can either concentrate light or distort a reflection depending on the distance of the observer.