Hybrid electric vehicle (HEV)
A hybrid electric vehicle (HEV) is a hybrid vehicle that combines a conventional propulsion system with a rechargeable energy storage system (RESS) to achieve better fuel economy than a conventional vehicle. Its secondary propulsion system, additional to the electric motors, means that it does not require regular visits to a charging unit as a battery electric vehicle (BEV) does.
Modern mass-produced HEVs prolong the charge in their batteries by capturing kinetic energy by means of regenerative braking, and some HEVs can use the internal combustion engine (ICE) to generate electricity by spinning an electrical generator (often a motor-generator) to either recharge the battery or directly feed power to an electric motor that drives the vehicle. Many HEVs reduce idle emissions by shutting down the ICE at idle and restarting it when needed (start-stop system). An HEV’s engine is smaller than a non-hybrid fossil-fuel vehicle (petroleum fuel vehicle) and may be run at various speeds, providing greater efficiency.
(text source: http://en.wikipedia.org/wiki/Hybrid_electric_vehicle )