tracer

Từ điển American Heritage Dictionary 4th

trac·er (trāʹsər)n.
1.
a. One who is employed to locate missing goods or persons.
b. An investigation or inquiry organized to trace missing goods or persons.
2. Any of several instruments used in making tracings or in imprinting designs by tracing.
3. A tracer bullet.
4. An identifiable substance, such as a dye or a radioactive isotope, that is introduced into a biological or mechanical system and can be followed through the course of a process, providing information on the pattern of events in the process or on the redistribution of the parts or elements involved. Also called label.