An abnormal increase in the number of cases of a disease in a population is referred to as a/an:

An abnormal increase in the number of cases of a disease within a specific population is referred to as an epidemic. This term indicates that the disease is occurring more frequently than expected within that geographic area or population group. Epidemics can arise from various factors including an introduction of a new pathogen, changes in host susceptibility, environmental changes, or a breakdown in public health measures.

While a pandemic refers to an epidemic that has spread over a larger area, often crossing international boundaries and affecting a large number of people, it does not apply to localized increases in disease cases. An endemic is a disease or condition regularly found among particular people or in a certain area, typically at a stable rate. An outbreak refers to a sudden rise in the number of cases but could be limited in scope, similar to an epidemic but usually on a smaller scale.

Therefore, the correct terminology for an abnormal increase in cases qualifying as an epidemic is specifically tailored to indicate significant fluctuations in disease incidence compared to what is considered normal or expected for that region or population at a given time.