Incidence refers to the number of people affected at a given time in a particular area.
Other /More definition:
Incidence refers to the frequency with which something, such as a disease or crime appears in a particular population or area. In disease epidemiology, the incidence is the number of newly diagnosed cases during a specific time period. The incidence is distinct from the prevalence which refers to the number of cases alive on a certain date.
Incidence figures allow authorities to determine whether the rate of new cases is stable or changing from one time period to the next.