Worldkiller Missile

A common name for every nuclear missile simply too strong to have any sort of use in space combat. After all it isn't impossible to construct a 10 gigatonne nuclear warhead, but in the world where anti-rocket defense is so strong you need a lot of missiles to break through enemy defense, and it's much more economical to create a thousand 10 megatonne warheads.
Such gloriously overpowered warheads can be, however, used against targets that lack anti-rocket defense, for example planets. A habitable planet hit by a 10 gigatonne nuclear warhed will be anything but habitable for a long while, and that's in case it doesn't get knocked out of its stable orbit and goes who knows where.
It is pretty much a poor man's choice in terms of worldkilling wepaonry, since it's pretty primitive and lacks the precision of a Lighthammer, while still being at least theoretically possible to be stopped unlike the Relativistic Bombardment.