tracer Definition

1a. Someone whose job is to trace, eg architectural, civil engineering, etc drawings;
1b. A device that traces.
2. A bullet, shell, etc which leaves a smoke-trail behind it so that its flight path can be seen.

Form: tracer bullet (in full)
Form: tracer shell
3. A substance, especially a radioactive element, whose course through the body, or effect on it, can be observed.