an investigator who is employed to find missing persons or missing goods
"The police hired a private tracer to locate the stolen jewelry before it crossed the border."
an instrument used to make tracings
"The architect placed a sensitive tracer over the original blueprint to copy every curve and line for the new construction drawings."
(radiology) any radioactive isotope introduced into the body to study metabolism or other biological processes
"The doctor injected a tracer into my bloodstream so they could monitor how quickly my liver was processing the medication."
ammunition whose flight can be observed by a trail of smoke
"The sniper fired a tracer round, and we could see its path glowing through the dark night as it streaked toward the target."
A compound, element, or isotope used to track the progress or history of a natural process.
"Scientists injected a radioactive tracer into the soil to monitor how quickly water moves through the underground aquifer."
Tracer comes from the word trace combined with the suffix -er. It originally referred to something or someone that makes a trace.