two and a half rotations (a nine-hundred degree turn). usually this is done as a trick on a skateboard, bmx bike, or with in-line skates. if you're really insane, you'll pull it off in a car.
the first 900 on a skateboard was done by tony hawk.
an extremely difficult trick done on a skateboard where you do a mute grab and just spin backside until you don't even know which way is up. it's actually performed by adding a 540 to a 360; you have to wind up for the 360 while spinning the first 540. people don't realize how insane it really is.
tony hawk had to go the the chiropractor often while trying to learn the 9.