A weapon from the book Ender's Game, also called "Little Doctor" or MD Device. Hence its name - MD is an abbreviation for Medical Doctor, "Dr. Device" a pun. Here it stands for Molecular Detachment Device.
When fired at a target, it sets off a chain reaction that destroys it. From the destroyed target the MD effect spreads out in a spherical field. Anything that the expanding sphere runs into is destroyed as well, sending out a new sphere, fueling the chain reaction with its mass. If the field does not encounter any new matter to tear apart, it will eventually weaken and dissipate. The matter the field reacted with is still mostly there when the effect is over, but its structure is completely destroyed - its just a lump of mud now.
With such a weapon it is possible to destroy targets of any scale, and of any number if they are grouped together closely enough, with one shot. It is therefore capable of completely destroying entire planets, or even suns and other stellar phenomena. Of course this weapon is only practical in space. When used in a planetside attack it would ignite the atmosphere and destroy the attacker as well.
How does it work? The Dr. Device is a work of fiction - the only information about its operating principle that can be found in Ender's Game is the following:
"At the focal point of two beams, it sets up a field in which molecules can't hold together anymore. Electrons can't be shared."
Strangely the Dr. Device is a beam type of weapon in Ender's Game, but a missile in Xenocide (which is one of the sequels).
The idea for the Dr. Device possibly came from a quite widespread fear from before the first atomic bomb was ignited: It was thought possible that the nuclear chain reaction from the warhead might not stop at the uranium, but go on forever, ignite the atmosphere and blow up the world. Luckily that is not the case.