Source : Webster's Revised Unabridged Dictionary (1913)

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

   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.

Source : WordNet®

indicator
     n 1: a number or ratio (a value on a scale of measurement)
          derived from a series of observed facts; can reveal
          relative changes as a function of time [syn: {index}, {index
          number}, {indicant}]
     2: a signal for attracting attention
     3: a device for showing the operating condition of some system
     4: (chemistry) a substance that changes color to indicate the
        presence of some ion or substance; can be used to indicate
        the completion of a chemical reaction or (in medicine) to
        test for a particular reaction