Okay here is the definition I have found.
A streetfighter is a sport bike that is customized by removing the fairing, and making other changes that result in an overall more aggressive look. Beyond simply removing fairings, specific changes that exemplify the streetfighter look are a pair of large, round headlights, tall, upright handlebars such as those on a motocross bike, and short, loud, lightweight mufflers. Streetfighters is also the name of a UK motorcycle magazine.
Later streetfighters used custom-built frames intended to overcome the weakness of the tubular steel frames of the early 4-cylinder superbikes of the 1970s and '80s. Many of these frames turned out to be "beautifully crafted pieces of metallurgical art," perhaps only unintentionally. Many were also originally racing machines.
Made popular by European riders, this type of custom motorcycle gained popularity all over the world, and motorcycle manufacturers began responding in the late 1990s by adopting the terminology and producing factory built streetfighters, beginning with the 1994 Triumph Speed Triple and the 1999 Honda X11, up through the 2009 Ducati Streetfighter.