the sport of racing motorcycles over rough land, up hills, through streams etc (motor + cross-country). Form of motorcycle racing in which cyclists compete on a closed course marked out over natural or simulated rough terrain. Courses vary widely but must be 1.5-5 km (1-3 mi) in length, with steep inclines, hairpin turns, and mud. Motorcycles are grouped into classes according to engine displacement (e.g., 125-, 250-, 500-cc). It is probably the most physically demanding motorcycle sport