Tracer /(?)/

Tra·cer

Tracer

n.
  1. One who, or that which, traces.
  2. A person engaged (esp. in the express or railway service) in tracing, or searching out, missing articles, as packages or freight cars.
  3. An inquiry sent out (esp. in transportation service) for a missing article, as a letter or an express package.
  4. 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. (Mil.)
  5. the chemical substance used in tracer ammunition to cause it to be visible in flight. (Mil.)
  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.