noun

1. a person or thing that indicates; specif.,
   1. any device, as a gauge, dial, register, or pointer, that measures or records and visibly indicates
   2. an apparatus that diagrams the varying fluid pressure of an engine in operation
2. any of various substances used to indicate the acidity or alkalinity of a solution, the beginning or end of a chemical reaction, the presence of certain substances, etc., by changes in color
3. a statistic, as the unemployment rate, used to measure economic or social conditions
4. Ecol. a species of plant or animal, or a community, whose occurrence serves as evidence that certain environmental conditions exist