The Collaborative International Dictionary of English v.0.48:

Incidence \In"ci*dence\, n. [Cf. F. incidence.]
   [1913 Webster]
   1. A falling on or upon; an incident; an event; an
      occurrence. [Obs.] --Bp. Hall.
      [1913 Webster]

   2. (Physics) The direction in which a body, or a ray of light
      or heat, falls on any surface.
      [1913 Webster]

            In equal incidences there is a considerable
            inequality of refractions.            --Sir I.
                                                  Newton.
      [1913 Webster]

   3. The rate or ratio at which something occurs; as, the
      incidence of murder in Los Angeles; the incidence of
      cancer in men over 50.
      [PJC]

   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.
      [1913 Webster]