RhymeZone: tracer definitions

Definitions of tracer:

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

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See tracer used in context: 4 rhymes, several books and articles.