An outbreak is an epidemiological term used to describe the sudden occurrence of a disease or condition in patterns greater than those expected in a specific geographical area at a specific time. The term is not exclusively used for infectious diseases, e.g. in 1986, the Chernobyl nuclear reactor accident may have caused an outbreak of thyroid disease. This would be an example of an event that caused a common source outbreak.

Common source outbreaks are further categorized as point source, continuous or intermittent depending upon certain characteristics. Some sources use the term outbreak as if it is nearly synonymous with epidemic .