It’s a catch-all term, for sure. Traditional sports like gymnastics are normally not included no matter how extreme they are in reality.
Some folks posted a list on Wikipedia.
Included are Free-diving, Freestyle skiing, Parachuting, Skateboarding, Snowboarding, Aggressive skating, BASE jumping, Freestyle skiing, Roller skating, Sandboarding, Skateboard and Zorbing.
But what are Elevator surfing and Extreme ironing?
zorbing in Australia
Gymnastics Coaching.com 