BMX 
(bē′ĕm′ĕks)
(bē′ĕm′ĕks)n.
1. A bicycle race on a dirt track with steep hills and sharp curves.
2. A bicycle with a stout frame, knobby tires, and usually 20-inch wheels, used for racing on such a track and for performing tricks.
[Abbreviation and respelling of bicycle motocross using the letter X to represent the word cross.]