in·di·ca·tor (ĭnˈdĭ-kāˌtər)

1. One that indicates, especially a pointer or an index.
2. An instrument used to monitor the operation or condition of an engine, an electrical network, or another physical system; a meter or gauge.
3. The needle, dial, or other registering device on such an instrument.
4. 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 substances by means of a characteristic change, especially in color.