For four-wheeled vehicles (and by extension, vehicles with six, eight, or more wheels) this term is used to describe vehicles that are able to power at most two wheels, referred to as either front or rear-wheel drive. The term 4x2 is also used, to denote four total wheels with two being driven. Most road vehicles use a 2WD layout due to its light weight and simplicity. Traction on the road is usually sufficient that the driving force can be reliably transmitted through only two wheels.
For vehicles that have part-time four-wheel drive, the term refers to the mode when 4WD is deactivated and power is applied to only two wheels.
For two-wheeled vehicles such as motorcycles and bicycles, the term is used to describe vehicles which can power the front as well as the back wheel. The term 2x2 is also used to denote two total wheels with both being driven. 2x2 vehicles are typically either mechanically driven via chain or shaft or are hydraulic driven. This scheme greatly improves offroad performance, but is quite complicated and requires more power to operate, thus most 2WD machines are either "exotic" bikes for enthusiasts or created with special uses in mind.
Two-wheeled drive with sidecars
For three-wheeled vehicles such as motorcycles with sidecars, the term is used to describe vehicles which can power the sidecar as well as the back wheel. Sidecar-drive vehicles are typically mechanically driven via a shaft and may or may not have a differential. This scheme greatly improves offroad performance, but is more complicated and requires more power to operate, thus most 2WD machines are vehicles created with special uses in mind such as trials or military use. The first use of sidecar drive appears in 1928 with the apparent independent invention of Baughan in the UK, and Mokharov in the USSR.