(radiology) any radioactive isotope introduced into the body to study metabolism or other biological processes WordNet 3.0 & Open English WordNet

A radioactive tracer, radiotracer, or radioactive label is a chemical compound in which one or more atoms have been replaced by a radionuclide so by virtue of its radioactive decay it can be used to explore the mechanism of chemical reactions by tracing the path that the radioisotope follows from reactants to products. Wikipedia

A substance containing a radioisotope that is used for tracking purposes Wikipedia Disambiguation

chemical compound Wikidata

A minute quantity of radioactive isotope used in medicine or biology to study the chemical changes within living tissues. OmegaWiki

A radioactive isotope which, when injected into a chemically similar substance, or artificially attached to a biological or physical system, can be traced by radiation detection devices. Wiktionary

A compound, element, or isotope used to track the progress or history of a natural process. Wiktionary