Desmodromic valve
Desmodromic valves are those which are positively closed by a cam and leverage system, rather than relying on the more conventional springs to close the valves.
This is in the context of internal combustion engines. The valves in question are the ones that allow air into the cylinder and (usually different ones) that allow exhaust gases out. This refers, for example, to the valve control system used in Ducati engines: both valve movements (opening and closing) are "operated". It is usual to say that action on the valve is positive in both cases, in other words, both strokes are controlled.
Desmodromic valve actuation has been applied to all but a few Ducati motorcycles. Two primary mechanical methods have been used to transfer timing information, from the crankshaft, to the camshaft and ultimately the levers or "rockers" and valves. Initially bevel-driven camshafts were used. This involved transferring the timing information via several bevel (part conical gears where the rotating axis' of the two lie on an angle - 90deg for example) gears and a shaft running on the outside of the engine block. Then at around 1977, Chief Design Engineer Fabio Taglioni completed and tested an actuation system that used rubberized metal belts with timing teeth. These teeth would mesh with timing pulleys, also external to the main engine block, and transmit the timing information to the valves.
The primary reason for Desmodromic (Colloquial - "Desmo") systems is to improve valve timing at higher engine revolutions. On very high performance valve spring engines, the spring does not always have time to return to its pre-compressed position, causing the camshaft to recompress the spring and valve prematurely. This is called "valve float". The Desomodromic system also eliminates the extra "work" spent by the motor to open spring actuated valves. Therefore giving more actual power the the wheel rather than using it to work against the seat pressure on the spring.
There is however a down side to this system. The timing of the valve in regards to its opening and closing is governed by a belt-driven cam. If this belt fails the valves will not close in turn causing the piston to "crash" into the valve, a catastrophic failure. Because of this, Ducati recommends a rigorous valve adjustment every 6,000 miles.
In general mechanical terms, the word desmodromic is used to refer to mechanisms that have different controls for their actuation in different directions. It is derived from two Greek roots, desmos (controlled, linked) and dromos (course, track).