BMX Definition
bēĕmĕks
noun
A bicycle race on a dirt track with steep hills and sharp curves.
American Heritage
A bicycle with a stout frame, knobby tires, and usually 20-inch wheels, used for racing on such a track and for performing tricks.
American Heritage
Origin of BMX
Abbreviation and respelling of bicycle motocross using the letter X to represent the word cross
From American Heritage Dictionary of the English Language, 5th Edition