Definition of tracer noun from the Oxford Advanced American Dictionary

     

    tracer

     noun
    noun
    NAmE//ˈtreɪsər//
     
     
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  1. 1 a bullet or shell (= a kind of bomb) that leaves a line of smoke or flame behind it tracer bullets
  2. 2(technology) a radioactive substance that can be seen in the human body and is used to find out what is happening inside the body Radioactive tracers have been used to map the metabolic activity of the brain.
See the Oxford Advanced Learner's Dictionary entry: tracer