Indicator (noun)

1. A thing or person that shows or signifies the existence or presence of something else.
2. A device or substance used in scientific or technical apparatus to show the presence or level of a particular substance or condition.

Origin:

Late middle english (denoting a chemical substance used to test the presence of another substance): from latin indicator, from indicare "point out".

Examples:

1. The increase in unemployment is a clear indicator of the failing economy.
2. PH indicator paper is used to test the acidity or alkalinity of a solution.
3. The stock market is an important indicator of the health of the economy.
4. A red traffic light is an indicator to stop.
5. The needle on the gauge was an indicator of the amount of fuel in the tank.