Indicator


 * 1) In chemical analysis, a substance that changes color within a certain definite range of pH or oxidation potential, or in any way renders visible the completion of a chemical reaction; e.g. phenolsulfonphthalein
 * 2) An isotope that is used as a tracer
 * 3) The labeled substance whose distribution between reactants of a system is used to determine the amount of analyte present