The Collaborative International Dictionary of English v.0.48:

Detector \De*tect"or\, n. [L., a revealer.]
   One who, or that which, detects; a detecter. --Shak.
   [1913 Webster]

         A deathbed's detector of the heart.      --Young.
   [1913 Webster]

   2. 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.
          [Webster 1913 Suppl.]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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: detector, sensor, sensing
         element]
    2: rectifier that extracts modulation from a radio carrier wave
       [syn: detector, demodulator]
    3: electronic equipment that detects the presence of radio
       signals or radioactivity

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "detector":
   calorimeter, chemical detector, clinical thermometer, cryometer,
   electric thermometer, electronic counter, electronic detector,
   error corrector, error detector, galvanothermometer,
   gas thermometer, liquid-level sensor, meat thermometer,
   metal detector, photoelectric sorter, pyrometer, pyrometric cone,
   resistance pyrometer, sensor, telethermometer, temperature sensor,
   thermocouple, thermoelectrometer, thermometrograph, thermostat,
   ultrasonic detector