Popular slang1 for those athletic activities that necessarily involve going downhill or, in extreme cases, just going down. Includes, but is not limited to, mountain biking, alping skiing, skydiving, bungee jumping, snowboarding, and cliff diving. Easily distinguished from most other sports in that any sane human being would categorize a participant as "stark raving nutters" for wanting to do this every week.
1 Well, okay, my slang.