tracer

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. One who is employed to locate missing goods or persons.
2. n. An investigation or inquiry organized to trace missing goods or persons.
3. n. Any of several instruments used in making tracings or in imprinting designs by tracing.
4. n. A tracer bullet.
5. n. 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.

Century Dictionary and Cyclopedia

1. n. 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.
2. n. One who or that which traces, in any sense.
3. n. 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.
4. n. One whose duty it is to trace or search out missing articles, as railway-cars, milk-cans, or letters.
5. n. c) An inquiry sent out from a post-office, express-office, railway-station, or other establishment after some missing letter, package, car, etc.
6. n. One who copies or makes tracings of drawings, etc.
7. n. An instrument, like a stylus, for tracing drawings, etc., on superimposed paper.
8. n. A simple kind of pantograph.
9. n. 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.
10. n. 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.

Wiktionary

1. n. chemistry A chemical used to track the progress or history of a natural process.
2. n. 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.
3. n. The act of tracking or investigating something.
4. n. A person who traces something.

GNU Webster's 1913

1. n. One who, or that which, traces.
2. n. A person engaged (esp. in the express or railway service) in tracing, or searching out, missing articles, as packages or freight cars.
3. n. An inquiry sent out (esp. in transportation service) for a missing article, as a letter or an express package.
4. n. (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.
5. n. (Mil.) the chemical substance used in tracer ammunition to cause it to be visible in flight.
6. n. 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.

WordNet 3.0

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