"I shoulda known there was somethin' fishy goin' on then. He loved that motorbike, what was he givin' it ter me for? Why wouldn' he need it anymore? Fact was, it was too easy ter trace. Dumbledore knew he'd bin the Potters' Secret-Keeper. Black knew he was goin' ter have ter run fer it that night, knew it was a matter o' hours before the Ministry was after him."
-- Rubeus Hagrid (PA10)
The flying motorbike is an amazing Muggle machine was once owned by Sirius Black. It’s huge, big enough to carry Hagrid (or perhaps it magically expands as necessary to accommodate its rider (PS1, DH4)). Later Arthur Weasley got hold of it, and customized it still further (DH4). The motorbike served to transport Harry into Privet Drive after the death of his parents in 1981. Years later, it was used to transport Harry away from Privet Drive for the last time.
As a teenager, Sirius Black was a fan of Muggle motorbikes and had posters and photos of them on the walls of his room (DH10). He magically modified his own to be able to fly. This passion particularly annoyed his anti-Muggle family:
However, the most extreme anti-Muggles eschew all motorised transport; Sirius Black’s love of motorbikes incensed his hard-line parents (Pm).
Sirius Black and James Potter rode the motorbike while engaging with Death Eaters during their time with the Order of the Phoenix (Pre). After James was killed in Godric’s Hollow, Sirius arrived on the scene on the bike. When Hagrid also arrived with instructions from Dumbledore to take Harry to Privet Drive, Sirius lent him the bike to transport the child. After Hagrid delivered Harry to Dumbledore in Little Whinging, he returned the bike (PS1).
Over the years, the bike ended up in Arthur Weasley’s shed, where Arthur added a few enhancements, including a dragonfire accelerator and a device which emitted a magical wall to stop pursuers, as well as a sidecar. Hagrid flew the bike to Privet Drive as part of the Order of the Phoenix rescue mission to bring Harry to a safe house before he turned seventeen. Harry and Hagrid fought in the Battle of Seven Potters aboard the motorbike, but it was badly damaged when it crashed into the pond behind Ted Tonks’ house (DH4).
As part of Sirius Black’s possessions, the remains of the flying motorbike presumably belong to Harry now, but the fate of the damaged vehicle is not known.
The term 'motorbike' is used in the US to refer to a smaller, lightweight motorcycle. In Britain, however, the term is used for any motorcycle.
From the Web
Definition of 'motorbike' from the Cambridge Dictionary (click to see both American and British definitions)