Start-stop system
A fuel-saving feature that shuts the engine off when the car is stationary and restarts it when you release the brake or press the accelerator.
Idling at a red light burns fuel while the car goes nowhere. A start-stop system, also called idle-stop or smart-start, addresses that: fueleconomy.gov describes it as automatically turning the engine off when the vehicle comes to a stop and restarting it when the brake is released or the accelerator is pressed. The aim is to reduce wasted fuel from idling.
How much a car can save depends on how much time it spends stopped with the engine running, so a car that rarely idles has less to gain. Mild hybrids (MHEV) combine start-stop with regenerative braking, according to the same source, and full hybrids use both plus a larger battery and a more powerful electric motor.
Battery electric cars have no idling engine to switch off, so the concept does not apply to them.