Blind spot detection uses sensors to monitor the side of the vehicle for vehicles approaching blind spots. In many systems, a visual alert appears on or near the sideview mirrors if a vehicle is detected. An audible alert may activate if the driver signals a turn and there is a vehicle in the blind spot on the turning side. Some systems also may activate the brake or steering controls to keep the vehicle in its lane.

More information at http://www.iihs.org/research/qanda/crash_avoidance.aspx