Final answer:
The number of new cases of a disorder during a specific time period is referred to as incidence, which is distinct from prevalence that includes all cases.
Step-by-step explanation:
The number of new cases of a disorder that appear during a specific time period is called incidence. This measure is crucial for epidemiologists and public health officials because it helps in understanding risk factors and the spread of disease. Unlike prevalence, which includes both new and existing cases, incidence focuses solely on new cases and is typically expressed as a rate per 100,000 people. For example, when investigating the spread of a chronic disease in the southern United States and comparing data from different years, researchers would look at the incidence rates to determine if there is an increase in new cases over time.