Source : Webster's Revised Unabridged Dictionary (1913)

   3. (Mech.) The part of an instrument by which an effect is
      indicated, as an index or pointer.

   4. (Zo["o]l.) Any bird of the genus {Indicator} and allied
      genera. See {Honey guide}, under {Honey}.

   5. (Chem.) 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.

   Note: The common indicators are litmus, trop[ae]olin, phenol
         phthalein, potassic permanganate, etc.

   {Indicator card}, the figure drawn by an engine indicator, by
      means of which the working of the engine can be
      investigated and its power calculated. The Illustration
      shows one form of indicator card, from a steam engine,
      together with scales by which the pressure of the steam
      above or below that of the atmosphere, corresponding to
      any position of the engine piston in its stroke, can be
      measured. Called also {indicator diagram}.

   {Indicator telegraph}, a telegraph in which the signals are
      the deflections of a magnetic needle, as in the
      trans-Atlantic system.