Quiz: Remember the definition of mal de mer?
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.