Etymology
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tracer (n.)

c. 1500, "one who tracks or searches," agent noun from verb form of trace (n.1). Meaning "bullet whose course is made visible" is from 1910.

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Definitions of tracer

tracer (n.)
an investigator who is employed to find missing persons or missing goods;
tracer (n.)
an instrument used to make tracings;
tracer (n.)
(radiology) any radioactive isotope introduced into the body to study metabolism or other biological processes;
tracer (n.)
ammunition whose flight can be observed by a trail of smoke;
Synonyms: tracer bullet
From wordnet.princeton.edu