Webster's English Dictionary

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tracer

trac.er \'tra--s*r\ n 1a: a person who traces missing persons or property 
   and esp. goods lost in  transit 1b: an inquiry sent out in tracing a 
   shipment lost in transit  2: a draftsman who traces designs, patterns, or 
   markings  3: a device (as a stylus) used in tracing  4a: ammunition 
   containing a chemical composition to mark the flight of proj ectiles by a 
   trail of smoke or fire 4b: a substance and esp. a labeled element or atom 
   used to trace the course  of a chemical or biological process