any radioactive atom detectable in a material in a chemical, biological, or physical system and used to mark that material for study, to observe its progress through the system, or to determine its distribution. An isotopic tracer must behave as does the material being studied, but, in addition, it must have some distinguishing property by which it can be detected in the presence of the other material.
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|a scrap or morsel of food left at a meal.|
|a children's mummer's parade, as on the Fourth of July, with prizes for the best costumes.|