A polygon is concave if it has angles of greater than 180 degrees between consecutive sides, eg a dart or arrow-head shaped polygon is concave, but a pentagram is not, because although it is not convex, all of its five angles are less than 180 degrees Similarly, a polyhedron is concave if its dihedral angles are all less than 180 degrees Note that this is not quite the same as nonconvex
A surface that is concave curves inwards in the middle. a concave stomach. convex. a concave surface is curved inwards in the middle convex (concavus, from com- ( COM-) + cavus )
noun The inward curve, like a bowl, which a skateboard deck is given to provide more stability for the feet
Hollow and curved or rounded; vaulted; said of the interior of a curved surface or line, as of the curve of the of the inner surface of an eggshell, in opposition to convex; as, a concave mirror; the concave arch of the sky