A person or device that traces or tracks something; a bullet or shell containing a pyrotechnic charge that leaves a visible trail.
English from 'trace' + agent suffix '-er'. 'Trace' comes from Old French 'tracier' meaning to make one's way. The military sense of tracer bullets developed in the early 20th century, while medical tracers (radioactive isotopes) emerged later.
Tracer bullets were initially developed to help machine gunners see where their shots were going in the dark, but they inadvertently became crucial for aerial combat in World War I, allowing pilots to adjust their aim in real-time. The same principle now helps doctors trace biological processes using radioactive markers that 'light up' inside the body.
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