indicator

noun

1. 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
2. a signal for attracting attention
3. a device for showing the operating condition of some system
4. (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