tracer

noun

a person or thing that traces.
a person whose business or work is the tracing of missing property, parcels, persons, etc.
an inquiry sent from point to point to trace a missing shipment, parcel, or the like, as in a transportation system.
any of various devices for tracing drawings, plans, etc.
Also called tracer ammunition. ammunition containing a chemical substance that causes a projectile to trail smoke or fire so as to make its path visible and indicate a target to other firers, especially at night.
the chemical substance contained in such ammunition.
a substance, especially a radioactive one, traced through a biological, chemical, or physical system in order to study the system.

noun

a person or thing that traces
1. a projectile that can be observed when in flight by the burning of chemical substances in its base
2. ammunition consisting of such projectiles
3. (as modifier)tracer fire
med any radioactive isotope introduced into the body to study metabolic processes, absorption, etc, by following its progress through the body with a gamma camera or other detector
an investigation to trace missing cargo, mail, etc

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.

A substance, such as a dye or a radioactive isotope, that is introduced into and followed through a biological or chemical process, by virtue of its radioactive signature, color, or other distinguishing physical property, thus providing information on the course of the process or on the components or events involved.
An instrument used in dissecting nerves and blood vessels.

An identifiable substance, such as a dye or radioactive isotope, that can be followed through the course of a mechanical, chemical, or biological process. Tracers are used in radioimmunoassays and other laboratory testing. The use of radioactive iodine, for example, can give information about thyroid gland metabolism. Also called label