A set in 
is concave if it does not contain all the line
segments connecting any pair of its points. If the set does
contain all the line segments, it is called convex.
Concave
See also
Connected Set, Convex Function, Concave Polygon, Convex Hull, Convex Optimization Theory, Convex Polygon, Delaunay Triangulation, Simply ConnectedExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Concave." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Concave.html