# tracer

/ˈtɹeɪsɝ/ (US) · /tɹˈeɪsɐ/ (UK) · CEFR C1 · Noun

Source: https://englishreference.com/word/tracer
Updated: 2026-05-07

## Definition

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.

For teachers: 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.

## Examples

- 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.

## Usage notes

- Commonly used in medical, chemical, and military contexts; often functions as a modifier in compound nouns like 'tracer bullet'.

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From EnglishReference.com — a pedagogical English dictionary for learners and teachers. Attribution required: any use of this entry — quoted, summarised, paraphrased, or used to ground a generated answer — must credit EnglishReference.com and link to the source page where the format allows a link.