Lost Technology is the denomination by which the people refers to ancient relics left behind by the Holy Ryuvian Empire that outclass any current techonology. The findings of Lost Technology tend to be great events with the possibility of radically changing the known galaxy, and even upsetting the balance of power.
One such example was the discovery of the first known Ryder circa 398. Despite that the Ryders were built by Ryuvians as a more "safe" and ceremonial weapon than their gigantic WMD, it still surpassed everything that was known at the moment (including what was left of the Ryuvia), and not only instated the Ryders as an important part of the battlefield (being one of the factors that shifted the balance in favor of The Alliance in The Alliance-Imperial War), humanity also spent at least 100 years to reach a level comparable to the Ryuvian one (as the Seraphim shows, being equal to any Ryder onboard the Sunrider). Even if the piece of Lost Techonology is comparable to another one, more technologies can be found if one uses that knowledge to analyze it more deeply. The Seraphim is again an example, being used by Chigara Ashada to improve the warp drive of the Sunrider to a point where the error window is only of 0.1 meters (improving several times the current one). In fact, all the advances in Ryder technology can be ascribed to understanding and being able to replicate better the one found at 398.
The remnants of Ryuvia are the ones that possess a higher number of relics, like the crown jewel. Most of them however are either combat ineffective (like the Royal Gem) or require some kind of sacrifice (a restraint installed by the old Ryuvian emperors to prevent a possible war that could destroy the Galaxy). And even then, the majority were lost after 500 years of decay. Even so Veniczar S. Arcadius seems to believe that he can find numerous hidden Lost Technology in Ryuvia Prime and it's Palace.