- noun a substance inserted into an organism so that its movements may be tracked from its colour, radioactivity, fluorescence or other characteristic
- A radioisotope which substitutes a non-radioactive isotope of the same element, enabling the detection and monitoring which would otherwise be more difficult or impracticable. Used, for instance, in medical diagnostic techniques such as positron emission tomography. Also called radioactive tracer, or radiotracer.
- noun a substance, often a radioactive one, injected into a substance in the body, so that doctors can follow its passage round the body