n. (chemistry) a substance that changes color to indicate the presence of some ion or substance; can be used to indicate the completion of a chemical reaction or (in medicine) to test for a particular reaction

n. a number or ratio (a value on a scale of measurement) derived from a series of observed facts; can reveal relative changes as a function of time

n. a signal for attracting attention

n. an indicator in digital storage media used as a reference for different purposes