Weapon of Mass Destruction [WMD]

There was a time when a term 'Weapon of Mass Destruction' described weapons such as nuclear warheads or chemical weapons. In the space, when even simple cargo vessels can be used as devastating weapon capable of breaking apart a planet (all you have to do is make them go fast enough before ramming it), the term naturally grew and began describing weapons noticably stronger.
Today, the WMDs can be divided into two main categories. Fleetkillers and Worldkillers. The first includes weaponry mostly used to decimate entire fleets (up to the level of capital ships) with a single salvo. The latter includes weaponry that is mostly tailored with efficient orbital bombardment in mind. Former are typically more powerful and can be used to quickly de-inhabit whole planets if one wished so, however it is simply inefficient in terms of cost to quality.
The sheer firepower of them - especially fleetkillers - is so great that they are actually used rather rarely. They are weapons of strategic value, insanely costly and in most cases requiring Leviathan-size carriers, so it's rare for a country (other than superpowers) to have more than a single, and in most cases small, fleetkiller (and a bit more worldkillers).
And even it is used only as a last mean of defense, which is enough to become one of the factors behind current galactic-size stalemate that has frozen the Human-held part of Galaxy for several centuries already - after all, when a smaller country seems to be overwhelmed, it can almost always stop or at least temporarily halt it with a single fleetkiller armed colossi.
The supertech variants of both worldkillers and fleetkillers tend to be weird, even among other supertechs. The weirdness is showed up pretty well in their naming - the more poetic it is, the least is understood about it... in most cases to the point that there isn't even a scientific name for that, because there is no scientist that has even a faintest clue about the way they work.