a bullet or shell (= a kind of bomb) that leaves a line of smoke or flame behind it 曳光弹 tracer bullets 曳光弹。
- (specialist)
a radioactive substance that can be seen in the human body and is used to find out what is happening inside the body 示踪剂 Radioactive tracers have been used to map the metabolic activity of the brain. 放射性示踪剂已经被用来绘制大脑的代谢活动。