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.

1535–45; trace1 + -er1 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
tracer (ˈtreɪsə)
1.  a person or thing that traces
2.  a.  a projectile that can be observed when in flight by the burning of chemical substances in its base
 b.  ammunition consisting of such projectiles
 c.  (as modifier): tracer fire
3.  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
4.  an investigation to trace missing cargo, mail, etc

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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American Heritage
Medical Dictionary

tracer trac·er (trā'sər)

  1. 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.

  2. An instrument used in dissecting out nerves and blood vessels.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
tracer   (trā'sər)  Pronunciation Key 
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.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Encyclopedia Britannica


detectable substance added to a chemical, biological, or physical system to follow its process or to study distribution of the substance in the system. Tracer dyes have long been used to follow the flow of underground streams. Incendiary rounds included at intervals in a belt of machine-gun bullets make the paths of the bullets visible. In scientific work, the use of tracers has increased and, because of the sensitivity of modern methods, has helped solve many problems. Particularly effective modern methods utilize isotopic tracers. See isotopic tracer.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
In all cases, turbulence in the fluid tends to diffuse, or mix out the tracer.
At dusk, a single tracer bullet was seen fired from the lifeboat.
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