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.