tracer

 
Pronunciation: /ˈtreɪsə/

noun

  • a person or thing that traces something or by which something may be traced, in particular:
  • a bullet or shell whose course is made visible in flight by a trail of flames or smoke, used to assist in aiming: the night sky turned into a light show of artillery, flares, and tracers [mass noun]: the night was lit up by tracer [as modifier]: tracer fire
  • a substance introduced into a biological organism or other system so that its subsequent distribution may be readily followed from its colour, radioactivity, or other distinctive property: they used radioactive carbon as a tracer [mass noun]: transfer of tracer from spinal fluid to blood
  • a device which transmits a signal and so can be located when attached to a moving vehicle or other object.