An Indicator

A pointer or index that indicates something.
A meter or gauge.
The needle or dial on such a meter.
(chemistry) Any of many substances, such as litmus, used to indicate the concentration of a substance, or the degree of a reaction.
(ecology) A plant or animal whose presence is indicative of some specific environment.
(economics) A measure, such as unemployment rate, which can be used to predict economic trends.
(Britain, Australia, automotive) A turn signal; each of the flashing lights on each side of a vehicle which indicate a turn is being made to left or right, or a lane change etc.
A bird, the honeyguide.