Shield X was an experimental Imperial deflector shield generator that afforded any starship powered by it near-invulnerability against standard attacks. Its only Con (as seen during fitting) is that any ship outfitted with the technology had to keep most of its energy focused on powering Shield X.
It was stolen by the Alliance to Restore the Republic during the Galactic Civil War. The CR90 corvette Blade 5 (later captured as Ram's Head) was outfitted with the device, and used during Operation Ram's Head. Its usage following that operation remains undocumented.
Some time later the Empire attempted to recreate the technology at a secret space installation. The base was destroyed by a Rebel B-wing strike force, and the project was supposedly stopped for good.