What is an indicator? how does its color change in an acidic environment?

An indicator is a substance that undergoes a distinct observable change when the conditions of the environment in which it is placed change. This could be a color change, sludge formation, bubble formation, temperature change, or some other measurable quality.

Another type of indicator that can be found in chemistry and other sciences is a pointer or light on a device or instrument that can indicate pressure, volume, temperature, etc. or the state of a piece of equipment (e.g. power on / off).