The Collaborative International Dictionary of English v.0.48:

Tracer \Tra"cer\, n.
   One who, or that which, traces.
   [1913 Webster]

   2. A person engaged (esp. in the express or railway service)
      in tracing, or searching out, missing articles, as
      packages or freight cars.
      [Webster 1913 Suppl.]

   3. An inquiry sent out (esp. in transportation service) for a
      missing article, as a letter or an express package.
      [Webster 1913 Suppl.]

   4. (Mil.) a type of ammunition that emits light or smoke as
      it moves toward its target, providing a visible path of
      the projectile in flight so that the point of impact may
      be observed; -- called also tracer ammunition.
      [PJC]

   5. (Mil.) the chemical substance used in tracer ammunition to
      cause it to be visible in flight.
      [PJC]

   6. a chemical substance with properties, such as
      radioactivity or fluorescence, which make it easily
      measurable, used to observe the movements of chemically
      related substances through a biological, physical, or
      chemical system; -- in biochemistry, also called labeled
      compounds.

   Note: Radioactive tracers are used, for example, to measure
         the retention or distribution of residues of drugs
         after administration to an animal, to determine the
         type and rate of metabolism; also, to measure the rate
         of motion of molecules in electrophoresis or the
         leakage of small quantities of material from a
         container. Small fluorescent tracers may be attached in
         many cases to macromolecules such as proteins or
         nucleic acids, allowing the motions of such
         macromolecules to be easily observed by their acquired
         fluorescence, without appreciably changing their
         properties. In biological and biochemial systems the
         common radioactive isotopes used in tracers are
         carbon-14, tritium (hydrogen-3), sulfur-35,
         phosphorus-32, and iodine-131; other isotopes are also
         used, including non-radioactive isotopes such as
         carbon-13.
         [PJC]