An Etymological Dictionary of Astronomy and Astrophysics

1. The degree, extent, or frequency of occurrence, especially of something unwanted.
   

2. The act of striking a → light ray or → beam of particles on a surface. Also the direction of striking.

Etymology (EN): M.E., from M.Fr. incidence, from L.L. incidentia, from incidere “to happen, befall,” from in- “on” + -cidere, combining form of cadere “to fall,” → case.

Etymology (PE): Fotâd,literally “fall, the act of falling,” from fotâdan “to fall,” variant of oftâdan “to fall; to happen,” → coincide.