indicator >> ˈɪndɪkeɪtə
Definition: [noun] a thing that indicates the state or level of something.
Example: car ownership is frequently used as an indicator of affluence
Definition: [noun] a gauge or meter of a specified kind.
Example: a speed indicator
Definition: [noun] a compound that changes colour at a specific pH value or in the presence of a particular substance, and can be used to monitor acidity, alkalinity, or the progress of a reaction.
Example: the remaining alkali is titrated against standard acid using phenolphthalein as indicator