Indicator

Part of speech: noun (n.)

1. One who, or that which, shows or points out; as, a fare indicator in a street car.

2. A pressure gauge; a water gauge, as for a steam boiler; an apparatus or instrument for showing the working of a machine or moving part; as:

3. A telltale connected with a hoisting machine, to show, at the surface, the position of the cage in the shaft of a mine, etc.

4. The part of an instrument by which an effect is indicated, as an index or pointer.

5. Any bird of the genus Indicator and allied genera. See Honey guide, under Honey.

6. That which indicates the condition of acidity, alkalinity, or the deficiency, excess, or sufficiency of a standard reagent, by causing an appearance, disappearance, or change of color, as in titration or volumetric analysis.