Source : Webster's Revised Unabridged Dictionary (1913)

Detector \De*tect"or\, n. Specifically:
   (a) An indicator showing the depth of the water in a boiler.
   (b) (Elec.) A galvanometer, usually portable, for indicating
       the direction of a current.
   (c) (Elec.) Any of various devices for detecting the presence
       of electric waves.

Detector \De*tect"or\, n. [L., a revealer.]
   One who, or that which, detects; a detecter. --Shak.

         A deathbed's detector of the heart.      --Young.

   {Bank-note detector}, a publication containing a description
      of genuine and counterfeit bank notes, designed to enable
      persons to discriminate between them.

   {Detector lock}. See under {Lock}.

Source : WordNet®

detector
     n 1: any device that receives a signal or stimulus (as heat or
          pressure or light or motion etc.) and responds to it in
          a distinctive manner [syn: {sensor}, {sensing element}]
     2: rectifier that extracts modulation from a radio carrier wave
        [syn: {demodulator}]
     3: electronic equipment that detects the presence of radio
        signals or radioactivity