What is a cancer cluster?

More than expected. A cancer cluster is a term used by statisticians and epidemiologists to refer to a greater than expected incidence of cancer in a certain group of people in the same geographic area over a certain period of time, often suggesting an environemental link. A notable historical example is testicular cancer among chimney sweeps in 18th century london.
See below. When a number of cases of cancer of the same type occur in a small area. See this for more info on this topic. http://www.cancer.gov/cancertopics/factsheet/risk/clusters.