indicator

(noun) 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. Synonyms: index, index number, indicant.

(noun) a signal for attracting attention.

(noun) a device for showing the operating condition of some system.

(noun) (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.