Definition of: indicator

(in′də·kā′tər) noun
1. One who or that which indicates or points out.
2. Any contrivance or apparatus, automatic or otherwise, which makes a mark, record, or sign to indicate the condition or position of something: a water gage; a speed indicator; a dial showing the position of elevators, cages, etc.; a device for recording the arrival and departure of trains, the number of fares collected in street cars, etc.
3. An instrument attached to a steam engine which, by the action of the steam itself, draws an indicator–diagram from which may be ascertained the gross power, the correct adjustment of the distribution valves, the ratio of the pressure in the cylinder to the boiler pressure, etc.
4. Chem. A substance, as litmus, potassium permanganate, etc., which colors a solution and by its disappearance, reappearance, or change of color when a reagent is added, indicates alkalinity, acidity, etc.
5. Ecol. A type of vegetation or plant growth serving to indicate the general character of a habitat. [<LL]