Definition of indicator

• n. - One who, or that which, shows or points out; as, a fare indicator in a street car.
• n. - 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
• n. - An instrument which draws a diagram showing the varying pressure in the cylinder of an engine or pump at every point of the stroke. It consists of a small cylinder communicating with the engine cylinder and fitted with a piston which the varying pressure drives upward more or less against the resistance of a spring. A lever imparts motion to a pencil which traces the diagram on a card wrapped around a vertical drum which is turned back and forth by a string connected with the piston rod of the engine. See Indicator card (below).
• n. - A telltale connected with a hoisting machine, to show, at the surface, the position of the cage in the shaft of a mine, etc.
• n. - The part of an instrument by which an effect is indicated, as an index or pointer.
• n. - Any bird of the genus Indicator and allied genera. See Honey guide, under Honey.
• n. - 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.