The Spatial Distortion Accelerator was the earliest type of hyperspace generator used to enable a large starship to enter or exit hyperspace. Fitted to the largest warships, such as the Coral Sea-class Star Control Ships and Nicodemus-class Dreadnoughts, it enabled them and their escorting vessels to enter the deep umbra and thus bypass petty reality-based problems such as the speed of light barrier.
Using a combination of cutting-edge particle physics and powerful correspondence magick, the device tears a hole in the very fabric of reality, a hole dozens of miles wide. The edges of the hole are dangerous zones of twisted spacetime, where steep gravity gradients and powerful electromagnetic fields would tear apart the very atoms of any object that strays too close. The jump point thus created cannot be formed within the hyper limit of a star or large planet, where the celestial body's own gravity well is a steep enough pucker in the fabric of reality to interfere with the process.
Behind the Scenes
How exactly any of this works has never been made clear. The term comes from Bax, where he got it, we're not sure. But we've had to twist the technobabble a lot to retcon it together with some of Martin's ideas on space travel.