- adjective curved inwards like the inner surface of a sphere
- adjective referring to a polygon with an interior angle greater than 180°
- Said of a curve that bulges away from some reference point, usually the horizontal axis or the origin of a diagram. More formally, a curve is concave from below (or concave to something below it) if all straight lines connecting points on it lie on or below it. Contrasts with convex.
- Having a surface, side, or boundary that curves inward, such as the inner surface of a sphere. This contrasts with convex, which has a surface or boundary that curves outward.
- adjective curving towards the inside