TRACER

noun Date: 14th century one that traces, tracks down, or searches out: as, a person who traces missing persons or property and especially goods lost in transit, an inquiry sent out in tracing a shipment lost in transit, one who traces designs, patterns, or markings, a device (as a stylus) used in tracing, 4. ammunition containing a chemical composition to mark the flight of projectiles by a trail of smoke or light, a substance used to trace the course of a chemical or biological process