A V12 is an engine in V configuration, having 12 cylinders.
A number of World War II fighterss and bombers used V12 engines such as the Rolls-Royce Merlin engine or the Allison V-1710, usually generating about 1,500 horsepower. Their use disappeared quickly after the advent of the jet engine.
In automobiles, 12-cylinders engines have never been common, their use mainly confined to expensive luxury and sporting cars.
They used to be common in Formula One and Endurance racing. Between 1965 and 1980 Ferrari, Weslake, Honda, BRM, Maserati, Matra, Alfa-Romeo and Tecno have used 12 cylinders in Formula One, either V12 or Flat-12, but the Ford Cosworth V8 had slightly better power-to-weight ratio and less fuel consumption, thus it was more successful despite being less powerful than the best V12. During the same era V12 engines where superior to V8 in Endurance racing, reduced vibrations giving better reliability. In the 1990s the Renault V10 engines proved their superiority against the Ferrari and Honda V12 and the Ford V8. Now all Formula One cars use V10 engines.
For production cars 12-cylinder engines can give superlative performance and smoothness characteristics, but only at the cost of significant additional mechanical complexity and cost. Before the World War II, V12 engines were used on some models from Cadillac, Packard, Lincoln, Rolls-Royce and Hispano-Suiza. Recent models from Aston Martin, Jaguar, Mercedes-Benz, BMW, Rolls-Royce, Lamborghini and Ferrari use V12 engines.
Mercedes (MTU) manufactured a line of V12 diesel engines for marine use. These engines commonly power craft up to about 100 tons in pairwise configurations. These large engines required about 35 gallons of oil in the crankcase.
Tatra also made a truck with two air cooled V12 engines. One engine on each side. The right engine powering the right wheels and the left engine powering the wheels on the left side.