The number of new cases of a disease among a certain group of people during a specific period of time. It is different from prevalence. (NICE)

Incidence is the rate of new cases or events over a specified period for the population at risk for the event. In medicine, the incidence is commonly the newly identified cases of a disease or condition per population at risk over a specified timeframe. An example of incidence would be 795,000 new strokes in the United States, annually. Here the incidence is 795,000 new strokes, the population in the United States, and the timeframe is one year. Alternatively, incidence can be specified as person-years... (StatPearls)