The Collaborative International Dictionary of English v.0.48:

Indicator \In"di*ca`tor\ ([i^]n"d[i^]*k[=a]`t[~e]r), n. [L.: cf.
   F. indicateur.]
   [1913 Webster]
   1. One who, or that which, shows or points out; as, a fare
      indicator in a street car.
      [1913 Webster]

   2. (Mach.) 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:
      (a) (Steam Engine) 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).
      (b) A telltale connected with a hoisting machine, to show,
          at the surface, the position of the cage in the shaft
          of a mine, etc.
          [1913 Webster]

   3. (Mech.) The part of an instrument by which an effect is
      indicated, as an index or pointer.
      [1913 Webster]

   4. (Zool.) Any bird of the genus Indicator and allied
      genera. See Honey guide, under Honey.
      [1913 Webster]

   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.
      [1913 Webster]

   Note: The common indicators are litmus, trop[ae]olin, phenol
         phthalein, potassic permanganate, etc.
         [1913 Webster]

   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.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Diagram \Di"a*gram\, n. [Gr. ?, fr. ? to mark out by lines; dia`
   through + ? to draw, write: cf. F. diagramme. See Graphic.]
   1. (Geom.) A figure or drawing made to illustrate a
      statement, or facilitate a demonstration; a plan.
      [1913 Webster]

   2. Any simple drawing made for mathematical or scientific
      purposes, or to assist a verbal explanation which refers
      to it; a mechanical drawing, as distinguished from an
      artistical one.
      [1913 Webster]

   Indicator diagram. (Steam Engine) See Indicator card,
      under indicator
      [1913 Webster]