noun
: a plane section through the cusp of a caustic surface that is visible on a plane surface where light has been reflected from a smooth concave surface (as the inside of a metal ring)
noun
: a plane section through the cusp of a caustic surface that is visible on a plane surface where light has been reflected from a smooth concave surface (as the inside of a metal ring)
Webster's New International English Dictionary. Новый международный словарь английского языка Webster. 2012