tracer
/ˈtreɪsə(r)/noun
1- a person or thing that traces something or by which something may be traced, in particular追踪者, 追踪装置, 尤指：
1.1
- a bullet or shell whose course is made visible in flight by a trail of flames or smoke, used to assist in aiming曳光弹。
1.2
- a substance introduced into a biological organism or other system so that its subsequent distribution may be readily followed from its colour, fluorescence, radioactivity, or other distinctive property示踪剂, 显光剂。
1.3
- a device which transmits a signal and so can be located when attached to a moving vehicle or other object跟踪器（安装在移动车辆上或其他物体上, 通过其发出的信号进行跟踪）。