tracer

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One who is employed to locate missing goods or persons.
• noun An investigation or inquiry organized to trace missing goods or persons.
• noun Any of several instruments used in making tracings or in imprinting designs by tracing.
• noun A tracer bullet.
• noun 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.

from The Century Dictionary.

• noun In ordnance, an attachment, to a projectile for use at night, provided with a burning composition by means of which, when the projectile is fired from a gun, its path throngh the air is rendered visible.
• noun One who or that which traces, in any sense.
• noun A small slender steel instrument, having a handle in the middle and its ends pointed more or less, and one of them usually also curved and edged, used in dissection as a compromise between scalpel and probe for tracing out the course of nerves, vessels, etc. It is usually held like a pen, and may be pushed into or drawn through tissue, as desired. Also called seeker.
• noun One whose duty it is to trace or search out missing articles, as railway-cars, milk-cans, or letters.
• noun c) An inquiry sent out from a post-office, express-office, railway-station, or other establishment after some missing letter, package, car, etc.
• noun One who copies or makes tracings of drawings, etc.
• noun An instrument, like a stylus, for tracing drawings, etc., on superimposed paper.
• noun A simple kind of pantograph.
• noun A form of outline-or copying-machine. It consists essentially of a long bar balanced by means of a universal joint near one end. The longer arm is directed toward the drawing, design, or other work to be copied on a reduced scale, and the shorter arm carries a pencil. On moving the point of the long arm over the work, the pencil on the short arm reproduces a reduced copy of the work on paper held before it. By reversing the relative positions of the pointer and pencil, an enlarged copy may be made. Also called tracing-machine.
• noun A tool, sometimes a small smooth-edged wheel set in a handle, by means of which a continuous line is impressed, as in ornamental metal-work.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who, or that which, traces.
• noun A person engaged (esp. in the express or railway service) in tracing, or searching out, missing articles, as packages or freight cars.
• noun An inquiry sent out (esp. in transportation service) for a missing article, as a letter or an express package.
• noun (Mil.) a type of ammunition that emits light or smoke as it moves toward its target, providing a visible path of the projectile in flight so that the point of impact may be observed; -- called also tracer ammunition.
• noun (Mil.) the chemical substance used in tracer ammunition to cause it to be visible in flight.
• noun a chemical substance with properties, such as radioactivity or fluorescence, which make it easily measurable, used to observe the movements of chemically related substances through a biological, physical, or chemical system; -- in biochemistry, also called labeled compounds.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun chemistry A chemical used to track the progress or history of a natural process.
• noun A piece 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 at night.
• noun The act of tracking or investigating something.
• noun A person who traces something.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (radiology) any radioactive isotope introduced into the body to study metabolism or other biological processes
• noun ammunition whose flight can be observed by a trail of smoke
• noun an investigator who is employed to find missing persons or missing goods
• noun an instrument used to make tracings