tracer

noun

1. A compound, element, or isotope used to track the progress or history of a natural process.
2. A round of ammunition for a firearm that contains magnesium or another flammable substance arranged such that it will burn and produce a visible trail when fired in the dark.
3. The act or state of tracking or investigating something.

   “We have a five-man tracer on him now. He's heading for a Vorster cell on Michigan Boulevard, and he's drunk as a lord. Should we intercept him?”

   “A surveyor typically conducts a tracer on his or her own and later meets up with the rest of the team to discuss findings.”

   “Ain't never running but I'm a paper chasing They gon catch up to me, they gon need a pacer How they over-watching me without a tracer”

4. A request to trace the movements of a person or an object, such as a shipment.

   “The next morning when she awoke, she told Darcy what had happened, and Darcy agreed that she should go to the police and this time ask them if they could put a tracer on her incoming phone calls, just in case he called again.”

5. A person who traces something.
6. A device or instrument used to assist in making tracings.
7. A process that traces something, such as the path of execution of a program during debugging.