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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.
incidence in·ci·dence (ĭn'sĭ-dəns)
The extent or rate of occurrence, especially the number of new cases of a disease in a population over a period of time.
The arrival of radiation or a projectile at a surface.