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indicator in·di·ca·tor (ĭn'dĭ-kā'tər)
One that indicates, especially a pointer or an index.
An instrument used to monitor the operation or condition of an engine, an electrical network, or another physical system; a meter or gauge.
The needle, dial, or other registering device on such an instrument.
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.
any substance that gives a visible sign, usually by a colour change, of the presence or absence of a threshold concentration of a chemical species, such as an acid or an alkali in a solution. An example is the substance called methyl yellow, which imparts a yellow colour to an alkaline solution. If acid is slowly added, the solution remains yellow until all the alkali has been neutralized, whereupon the colour suddenly changes to red