What does "incidence" mean?

Incidence: (Physics) The direction in which a body, or a ray of light or heat, falls on any surface. In equal incidences there is a considerable inequality of refractions. Sir I. Newton. Angle of incidence, the angle which a ray of light, or the line of incidence of a body, falling on any surface, makes with a perpendicular to that surface; also formerly, the complement of this angle. -- Line of incidence, the line in the direction of which a surface is struck by a body, ray of light, and the like.