a fact or trend that acts as a measure of the level or state of something, something that indicates some condition;
e.g. This index is an indicator of how wealthy the people of a country are, on average.
(noun)
a meter, dial, or gauge of some kind that provides the measurement of some quantity or indicates some information on a machine, especially a flashing light on a vehicle that acts as a signal when turning (in British usage);
e.g. The indicator on the tank said that the pressure inside wasn't too high. He used the indicator to signal that he was turning to the right.