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

Tracer
Martin visits his estranged wife`s sister in Florida. The two become lovers until Alex, the wife, arrives to get Martin back. Caught between the two women, he flees.

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1. \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.
   

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