A piece of glass with a reflective substance placed on the back of it which is used to reflect light.
In an automotive sense, the term "mirrors" usually refers to the side and review mirrors used by the vehicle driver to see behind without having to turn around. Mirrors can be manually operated or have electric motors which adjusts them so any driver can move them so they are useful. Mirrors help eliminate blind spots in driver's field of view. They also allow the driver to see most things behind them without the need to turn around and take their eyes off of the road.