1. 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.
2. 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.
3. Ecology An indicator species.
4. Any of various statistical values that together provide an indication of the condition or direction of the economy.
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