A trolleybus (also known as trolley bus, trolley coach, trackless trolley, trackless tram [in early years] or trolley is an electric bus that draws its electricity from overhead wires (generally suspended from roadside posts) using spring-loaded trolley poles. Two wires and poles are required to complete the electrical circuit. This differs from a tram or streetcar, which normally uses the track as the return part of the electrical path and therefore needs only one wire and one pole (or pantograph). They also are distinct from other kinds of electric buses, which usually rely on batteries.
More information on the Wikipedia page [1]
Trolleybuses served the London Passenger Transport Area for just over 30 years in the mid twentieth century. For much of its life, the London system was the largest in the world. It had 68 routes, and a maximum fleet of 1811 trolleybuses
The first 60 trolleybuses were operated by London United Tramways (LUT), from a depot at Fulwell in south-west London. They were nicknamed "Diddlers" and first ran on 16 May 1931,
More information on the Wikipedia page [2].
The London Transport Museum pages are [3] and [4].