An extreme sport (also called action sport and adventure sport) is a popular term for certain activities perceived as having a high level of inherent danger, and that are counter-cultural. These activities often involve speed, height, a high level of physical exertion, and highly specialized gear or spectacular stunts.
The definition of an extreme sport is not exact — for example, although studies show that (road) cycling ranks as the sport with the highest rate of injury, it is not considered an extreme sport because it is not counter-cultural. The term's origin is also unclear, but it gained popularity in the 1990s when it was picked up by marketing companies to promote the X Games
Wikipedia [accessed 10 May 2010] http://en.wikipedia.org/wiki/Extreme_sports