indicator

noun

1. 1

   a device for showing the operating condition of some system

2. 2

   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

3. 3

   a signal for attracting attention

4. 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