Concave means curved inward, or having negative curvature, as in certain mirrors and lenses; the word comes from the Latin concavus for "hollow." A biconcave lens has two concave surfaces, not necessarily with the same radii of curvature. A figure, such as a polygon or polyhedron, is said to be concave if a line segment joining any points inside the figure goes outside the figure. Similarly, a set is concave if it doesn't contain all the line segments connecting any pair of its points. Compare with convex.