the rate at which new cases of a disease occur in the population. The numerator is the number of new cases of a disease that occur during a given period (usually a year), and the denominator is the number of the population at risk of experiencing the disease during the same period, usually expressed as the number of new cases per 100,000 persons.
Notes:
This is the operational definition. It is commonly used even by international organizations in lieu of a more technically correct definition which is often not supported by readily-available data.