Urbandictionary.com says: A specific type of motorcycle, typically distinguished by it's [sic] aerodynamic 'hunched-over' seating position and high power to weight ratio. Often favoured by stunters, who choose the bikes because they are light and easy to perform tricks (such as wheelies and stoppies) on.

AKA, exceptionally sexy bikes that go really, really fast. :D Or, you know, rocket engines that you sit on. (Trust me, Tony Stark knows aaaaaalll about this concept...)