A pantograph (or "pan") is an apparatus mounted on the roof of an electric train, tram or electric bus to collect power through contact with an overhead catenary wire.
(source: Wikipedia)
For a clear example see post #393033 or post #1190447.
Note that this tag does not require the presence of any overhead wiring or even the vehicle itself in order to be used.
For a type of drafting instrument, use pantograph linkage.