While things are getting more sophisticated today, just as the uninformed call every four-wheel drive a Jeep, pretty much any customised bike was tagged as a chopper in the Sixties.
The movie 'Easy Rider' and Peter Fonda gave the chopper mainstream prominence and while, for many, it refers specifically to Harley-Davidsons, the term quickly evolved to be used loosely to refer to any custom, rather than to define a style of custom bike with a long, raked front end.
There are many kinds of choppers - from retros, which are usually built from used parts and handmade components, to choppers built with more standard methods and prefabricated aftermarket parts.
A chopper usually features radical styling and an unconventional build. It will generally have extended front forks, raised handlebars, lowered rear end, and can have all manner of exhaust pipes!
Initially, Harley-Davidson didn't like their engine parts being used, nor their bikes getting the chopper treatment, but they got over it and many classic choppers owe much to Harley-Davidson.
Of course, choppers have evolved to include not only raked frames, high handlebars, chrome wheels and exotic pipes, but now often feature exquisitely detailed mural paintwork, custom built frames and forks, exotic lighting, HUGE rear tyres & more.
Led by the likes of Orange County Choppers in the US, 'themed' choppers have been the hot trend in recent years, and have proved to be another source for some wild customs.