The term used to describe a group of fish is "school" or "shoal." When fish swim together in a coordinated manner, typically for protection or social interaction, they form a school. This behavior can help fish evade predators, optimize feeding opportunities, and improve reproductive success. Schools can vary in size, from just a few individuals to thousands, and they often exhibit synchronized movements. The shoaling behavior is crucial for many fish species, contributing not only to their survival but also to the functional dynamics of marine ecosystems.