An abreviation for the Executor class Star Dreadnought (Star Wars), commonly called a "super star destroyer".
A gigantic 17.6 km (11 mile) battleship, the first example, Executor, was Darth Vader's flagship during
the Empire Strikes Back.
The second SSD, Lusankya, was secretly buried under the surface of Coruscant, and later, under
the control of a cloned Palpatine, burst up out of
the planet and rose to space, killing some hundreds of millions in the process.