tracer

noun7

1. •
   A person who traces something.
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. •
   A compound, element, or isotope used to track the progress or history of a natural process.
4. •
   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? — 1964
5. •
   A request to trace the movements of a person or an object, such as a shipment.
6. •
   A process that traces something, such as the path of execution of a program during debugging.
7. •
   A device or instrument used to assist in making tracings.