1. the rate or range of occurrence or influence of something, especially of something unwanted: the high incidence of heart disease in men over 40.
  2. a falling upon, affecting, or befalling; occurrence: The incidence of murder that Sunday afternoon shocked the sleepy village.
  3. Optics, Physics.
    1. the striking of a ray of light, beam of electrons, etc., on a surface, or the direction of striking.
    2. angle of incidence(def 1).
  4. the fact or the manner of being incident.
  5. Geometry. partial coincidence of two figures, as of a line and a plane containing it.


  1. degree, extent, or frequency of occurrence; amounta high incidence of death from pneumonia
  2. the act or manner of impinging on or affecting by proximity or influence
  3. physics the arrival of a beam of light or particles at a surfaceSee also angle of incidence
  4. geometry the partial coincidence of two configurations, such as a point that lies on a circle

early 15c., “incidental matter,” from Middle French incidence (15c.), from Late Latin incidentia (see incident (n.)). Meaning “act of coming into contact with” is from 1650s; sense in physics is from 1620s.


  1. The extent or rate of occurrence, especially the number of new cases of a disease in a population over a period of time.
  2. The arrival of radiation or a projectile at a surface.
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