tracer
n
1 person or thing that traces 追踪者; 追踪装置; 描摹者; 描摹工具.
2 bullet or shell whose course is made visible by a line of smoke, etc left behind it 曳光弹
[attrib 作定语]
tracer bullets 曳光弹.
3 radioactive substance whose course through the human body, etc can be traced by the radiation it produces, used for investigating a chemical or biological process 示踪物.