1, The full form of DEMU is Diesel Electric Multiple Unit. It’s used on Transport & Travel ,Rail Transport in Worldwide

Diesel Electric Multiple Unit (DEMU) is a type of train powered by on-board diesel engines. DEMU runs on diesel as a fuel. The diesel engine powers an electrical generator or an alternator which produces electrical energy and subsequently feeds the electric motors.

A diesel multiple unit or DMU is a multiple-unit train powered by on-board diesel engines. A DMU requires no separate locomotive, as the engines are incorporated into one or more of the carriages. Diesel-powered single-unit railcars are also generally classed as DMUs.

A Diesel Multiple Unit (DMU) summons images of lightweight self-propelled railcars from Europe or Asia that require dedicated track.

In contrast, a locomotive is a powerful, heavy machine, a “can do” piece of equipment, that is at the core of an American train.
The US Railcar vehicle, was designed for North America starting with the horsepower of a small locomotive, using redundant engines, and the crash worthy framework required by the FRA to
run on freight track

2, The full form of DEMU is Diesel and Electric Modellers United. It’s used on Associations & Organizations ,Regional Organizations in United Kingdom

Diesel and Electric Modellers United (DEMU) is an independent society for modellers of the railways of mainland Britain in the Diesel and Electric era.