Source : Webster's Revised Unabridged Dictionary (1913)

Tracer \Trac"er\, n.
   1. A person engaged (esp. in the express or railway service)
      in tracing, or searching out, missing articles, as
      packages or freight cars.

   2. An inquiry sent out (esp. in transportation service) for a
      missing article, as a letter or an express package.

Tracer \Tra"cer\, n.
   One who, or that which, traces.

Source : WordNet®

tracer
     n 1: an investigator who is employed to find missing persons or
          missing goods
     2: an instrument used to make tracings
     3: (radiology) any radioactive isotope introduced into the body
        to study metabolism or other biological processes
     4: ammunition whose flight can be observed by a trail of smoke
        [syn: {tracer bullet}]