A multiple-unit train or simply multiple unit (MU) is a self-propelled train composed of one or more carriages joined together, which when coupled to another multiple unit can be controlled by a single driver, with multiple-unit train control.
Although multiple units consist of several carriages, single self-propelled carriages - also called railcars, rail motor coaches or railbuses - are in fact multiple-units when two or more of them are working connected through multiple-unit train control (regardless if passengers can walk between the units or not).
The term multiple unit does not denote locomotives using multiple-unit train control.