Once, a method of engine design. Today, a triumph of marketing.
Auto manufacturers (primarily the Japanese) appear to have convinced many people that DOHC (Dual OverHead Cam) engines are new, high-tech, and cool. In fact, many European manufacturers have used them for decades. I personally owned a 1974 Fiat (not exacly a luxury or exotic car) with a DOHC engine, and there was nothing esoteric about it.
There also seems to be a notion about that DOHC implies > 2 valves/cylinder. Again, nothing could be farther from the truth. The advantage of a DOHC engine is not the number of valves you can drive, but their placement. Even in a "conventional", 2-valve-per-cylinder engine, a DOHC design allows the valves to be placed in such a way that the flow of the gas-air mixture through the combustion chamber can be optimized.
The other advantages of a DOHC design are
- less friction than an OHV (pushrod) design,
- fewer moving parts than a pushrod design, and
- the ability (in some engines) to have different valve timing on the intake and exhaust valves.
It should be noted that there are really two "flavors" of DOHC design; in a four cylinder, 8-valve engine, typically one camshaft drives the intake valves and the other drives the exhaust valves. This scheme can be extended to 16-valve, 4-cylinder designs trivially. In a V-6, -8, or -12 engine with two banks of cylinders, each camshaft will generally drive all the valves in one bank of cylinders. This type of design might more properly be called a "double-SOHC" design, but in practice this distinction is very seldom made.