A bioindicator (also biological indicator) is a measure, an index of measures, or a model that characterizes an ecosystem or one of its critical components. It may reflect biological, chemical or physical attributes of ecological condition. The primary uses of an indicator are to characterize current status and to track or predict significant change. With a foundation of diagnostic research, an ecological indicator may also be used to identify major ecosystem stress.

(Adapted from U.S. Environmental Protection Agency.)