incidence

Origin:

late Middle English (denoting a casual or subordinate event or circumstance): from Old French, or from medieval Latin incidentia, from Latin incidere 'fall upon, happen to'(see incident). sense 1 dates from the early 19th century

Incidence and incidents sound the same, but incidence is more often used in technical contexts, referring to the frequency with which something occurs: increased ultraviolet light is likely to cause increased incidence of skin cancer. Incidents is simply the plural of incident, an event: the police are supposed to investigate any incidents of domestic violence. The form incidences should be avoided.