trac·er 
/ˈtreɪsɚ/ noun
plural trac·ers /ˈtreɪsɚ/ noun [count] 1 : a type of bullet that shows its path by creating smoke or light as it moves through the air
▪ The enemy fired tracers at the aircraft carrier. ▪ tracer fire/rounds/bullets 2 medical : a substance that is put in a patient's body so that doctors can watch a biological process (such as digestion) or see how a condition (such as cancer) is progressing
▪ They injected her with a radioactive tracer and tracked it via X-rays.
▪ The enemy fired tracers at the aircraft carrier. ▪ tracer fire/rounds/bullets
▪ They injected her with a radioactive tracer and tracked it via X-rays.
