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The Collaborative International Dictionary of English v.0.48 (gcide)

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]