one who, or that which, shows or points out; as, a fare indicator in a street car.-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-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).-a telltale connected with a hoisting machine, to show, at the surface, the position of the cage in the shaft of a mine, etc.-the part of an instrument by which an effect is indicated, as an index or pointer.-any bird of the genus indicator and allied genera. see honey guide, under honey.-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.