An electric trolleybus (or trolley bus) is a bus powered by two overhead electric wires, from which the bus draws electricity using two trolley poles (one for the return current, which can not pass to the ground as in the case of a tram). Unlike light rail, trolleybuses use rubber tires rather than steel wheels on rail.
Like other electric vehicles, trolleybuses are often seen as more environmentally friendly than hydrocarbon based vehicles such as buses, but the power is not "free", and instead has to be produced at centralised power plants.
One advantage that is rarely used in other vehicles is that they can generate electric power from kinetic power whilst braking, a process known as regenerative braking. Furthermore, trolleybuses demand a network of wires above the streets.