See indicator in Webster's New World College Dictionary
a person or thing that indicates; specif.,
any device, as a gauge, dial, register, or pointer, that measures or records and visibly indicates
an apparatus that diagrams the varying fluid pressure of an engine in operation
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
a statistic, as the unemployment rate, used to measure economic or social conditions
Ecol. a species of plant or animal, or a community, whose occurrence serves as evidence that certain environmental conditions exist
See indicator in American Heritage Dictionary 4
One that indicates, especially:
a. A pointer or an index.
b. An instrument used to monitor the operation or condition of an engine, furnace, electrical network, reservoir, or other physical system; a meter or gauge.
c. The needle, dial, or other registering device on such an instrument.
Chemistry Any of various substances, such as litmus or phenolphthalein, that indicate the presence, absence, or concentration of another substance or the degree of reaction between two or more substances by means of a characteristic change, especially in color.
Ecology A plant or animal whose existence in an area is strongly indicative of specific environmental conditions.
Any of various statistical values that together provide an indication of the condition or direction of the economy.