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.
