Outbreak vs. Cluster

Epidemic and outbreak are two terms that can actually be used interchangeably for the most part, although an outbreak is usually used to define a specific, smaller geographic area.

Cluster, on the other hand, often refers to outbreaks on small and specific scales. A cluster would occur over a specific time period, within a defined location. For example, a heightened number of cases in prison or long-term care facilities may be referred to as a cluster.

SOURCE: CDC