A "cluster" refers to a group of similar items or entities that are located close together. This term is commonly used in various fields, such as astronomy, where a cluster of stars can be found in a specific region of space, or in data analysis, where a cluster of data points may indicate a pattern or trend. In biology, a cluster can describe a group of cells or organisms that are found in proximity, such as a cluster of bacteria. Clusters can also be seen in social contexts, where groups of people may form clusters based on shared interests or characteristics, like a cluster of friends or colleagues.