1. An element or compound containing atoms that can be distinguished from their normal counterparts by physical means (e.g., radioactivity assay or mass spectrography) and can thus be used to follow (trace) the metabolism of the normal substances.
Origin: M.E. Track, fr. O. Fr. Tracier, to make one's way, fr. L. Traho, pp. Tractum, to draw, _ -er, agent suffix