Caustics (from the Greek kaustos, "burnt") are the envelopes of rays issued from a point (possibly infinite) and reflected from some curve. This name is explained by the fact that near caustic the intensity of the light field formed by the reflected (or diffused) rays sharply grows.

One caustic is certainly very well known to you. You can see it clearly every time you have your evening tea as a heart-shaped glowing trace on the cup’s surface. This is the cardioid, the envelope of rays reflected in a circle (of course, if the source of light is not in the center).