indicator

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

• a device for showing the operating condition of some system

• a signal for attracting attention

• 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

Related words

• Synonyms: 
  index , index number , indicant