"tracer" in the noun sense
1. tracer
an investigator who is employed to find missing persons or missing goods
2. tracer
an instrument used to make tracings
3. tracer
radiology) any radioactive isotope introduced into the body to study metabolism or other biological processes
4. tracer, tracer bullet
ammunition whose flight can be observed by a trail of smoke
Source: WordNet® (An amazing lexical database of English)
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