indicator

in·di·ca·tor

noun \ˈin-də-ˌkāt-ər\

Definition of INDICATOR

: a substance (as a dye) used to show visually usually by its capacity for color change the condition of a solution with respect to the presence of free acid or alkali or some other substance (as in detecting the end point of a titration) <litmus and phenolphthalein are acid-base indicators>