Literally, a falling on; whence, an accident or casualty.

The manner of falling on, or the direction in which one body falls on or strikes another. The angle which the line of falling, or the direction of a moving body striking another, makes with the place struck, is called the angle of incidence. When rays of light striking a body are reflected, the angle of incidence and the angle of reflection are equal. In equal incidences there is a considerable inequality of refractions.