tracer
n. countablen. a substance or object that scientists use to follow the movement of something through a system. It can be a chemical in the body or a special type of bullet that leaves a trail of light.
n. an identifiable substance, such as a radioactive isotope or fluorescent dye, introduced into a biological or mechanical system to observe its progress or distribution. Also refers to a projectile containing a pyrotechnic charge that burns during flight to reveal its trajectory.
The doctor used a radioactive tracer to find the blockage.
Engineers added a chemical tracer to the water supply to detect leaks in the underground pipe network.
By injecting a metabolic tracer into the bloodstream, researchers can monitor how different regions of the brain consume glucose during complex cognitive tasks.
From trace + -er.
Commonly used in medical, chemical, and military contexts; often functions as a modifier in compound nouns like 'tracer bullet'.