Action sports, adventure sports or extreme sports are activities perceived as involving a high degree of risk of injury or death. These activities often involve speed, height, a high level of physical exertion and highly specialized gear. Extreme tourism overlaps with extreme sport.

There is no precise definition of an 'extreme sport' and the origin of the term is unclear but it gained popularity in the 1990s when it was picked up by marketing companies to promote the X Games and when the Extreme Sports Channel and Extreme International launched. More recently, the commonly used definition from research is "a competitive (comparison or self-evaluative) activity within which the participant is subjected to natural or unusual physical and mental challenges such as speed, height, depth or natural forces and where fast and accurate cognitive perceptual processing may be required for a successful outcome" by Dr. Rhonda Cohen (2012).

While the use of the term "extreme sport" has spread everywhere to describe a multitude of different activities, exactly which sports are considered 'extreme' is debatable. There are, however, several characteristics common to most extreme sports. While they are not the exclusive domain of youth, extreme sports tend to have a younger-than-average target demographic. Extreme sports are also rarely sanctioned by schools for their physical education curriculum. Extreme sports tend to be more solitary than many of the popular traditional sports (rafting and paintballing are notable exceptions, as they are done in teams).

Activities categorized by media as extreme sports differ from traditional sports due to the higher number of inherently uncontrollable variables. These environmental variables are frequently weather and terrain-related, including wind, snow, water and mountains. Because these natural phenomena cannot be controlled, they inevitably affect the outcome of the given activity or event.

In a traditional sporting event, athletes compete against each other under controlled circumstances. While it is possible to create a controlled sporting event such as X Games, there are environmental variables that cannot be held constant for all athletes. Examples include changing snow conditions for snowboarders, rock and ice quality for climbers, and wave height and shape for surfers.

Whilst traditional sporting judgment criteria may be adopted when assessing performance (distance, time, score, etc.), extreme sports performers are often evaluated on more subjective and aesthetic criteria. This results in a tendency to reject unified judging methods, with different sports employing their own ideals and indeed having the ability to evolve their assessment standards with new trends or developments in the sports.

The origin of the divergence of the term "extreme sports" from "sports" may date to the 1950s in the appearance of a phrase usually, but wrongly, attributed to Ernest Hemingway. The phrase is;

There are only three sports: bullfighting, motor racing, and mountaineering; all the rest are merely games.