# concave

Concave mirror.

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.