LosTech

LosTech simply put refers to Lost Technology. Methods and Machines which were once in regular use but to which the methods of their manufacture have been in one way or another forgotten. The Term has it's origins in Battletech but the basic concept applies to various franchises.

Battletech

After the Fall of the Star League, the Five Great Houses decided to have one hell of a fight deploying their fleets against each others worlds and they were not shy about using Nuclear Weapons to get their way. If X planet in the Capellan Confederation had a Battlemech Factory that could make mechs to invade your worlds with which you could not quite capture, you blew it up. Same goes for a shipyard that can make WarShips. Or a factory which made high end manufacturing equipment for shipyards or Battlemech Factories. This was made all the worse as ComStar deliberately sabotaged things or assassinate experts under Operation Holy Shroud to prevent the development of certain capacities to maintain a balance of power. The fact most Great Houses instinctively blamed each other provided the supposedly neutral ComStar all the cover they needed.

By 3025 many of the Marvels of the Star League had become relics of a different time. The capacity to manufacture Compact K-F Drives was lost and with it the ability to make WarShips while the ability to make new regular JumpShips was substantially diminished. Items such as Gauss Cannons and Double Heat Sinks became irreplaceable. It was not until the discovery of the Helm Memory Core by merc’s (of all people) that Star League era tech began to be rediscovered and buildable again. Meanwhile, The Clans retained the ability to manufacture these items along with some new ones when they returned to the Inner Sphere in 3050.

Warhammer 40,000

In Warhammer 40,000, LostTech is another term for Archeotech, which shares the same basic idea; hyper-advanced technology left over from the Dark Age of Technology, where humanity had attained superscience that let them hold off the pre-Fall Eldar.

Real Life

• Concrete: Various forms of Concrete using (among other things) cement and volcanic ash were widely made during the Roman Empire, but the methods of producing these were lost in previous centuries. Turns out the secret to Roman Concrete was seawater interacting with silicon crystals to plug in any of the micro-cracks that would initially form after the concrete had cured. This is why you see ancient concrete buildings standing for 2000+ years near the sea while buildings made form modern/non-roman concrete begin to rot after ~20-40 years.
• Plumbing: Some Four Thousand Years ago the Bronze Age Indian City of Mohenjo Daro had a complex sewer system. During the Classical Era it became the norm for cities to have sewers and during the Roman Empire public water supplies were fairly common with fountains and private plumbing for the rich. After the collapse of the Roman Empire, both would fade away in Western Europe, only re-emerging in the Renaissance and the Age of Enlightenment.
• Greek Fire: A napalm-like substance that was often used by the Byzantine Empire in naval warfare by shooting it out onto enemy boats. It's notable for still burning on the surface of water. Still hotly debated as to what it's made of, most experts agree it's got Petroleum as a base ingredient, and something else added to make sure it stays thick and goopy.
• Damascus Steel: A rare example of modern LosTech being revived, just in a different way. A notably tough form of steel used in swords that has a cool wavy surface and was legendarily strong and flexible for the time they were made, giving rise to all sorts of tall tales as to what it can do from staying sharp for hundreds of years to cutting through the barrel of rifles. While the original techniques to make it were lost when the British Raj took over India, the actual metal itself (called Wootz) never disappeared and subsequent lab tests have created Damascus Steel so similar to the original you'd have to use a microscope to tell the difference, and arguably trying to reverse-engineer it created modern metallurgy.
• Clockwork mechanisms: In the period between 60-150 BC there existed at least one clockwork orrey (Antikythera Mechanism) which could calculate various astrological phenomena tied to the Moon and the Sun and there are numerous mentions of similar mechanisms in the collections of various wealthy individuals. After the Classical Era it would take over a thousand years for the technology incorporating clockwork to start developing again. The reason for this was that the market for such devices was small and the people who made them were few in number and guarded the secrets of their trade carefully, being either self-taught or trained one-on-one by an established master. When these rare expensive mechanisms broke down, if they could not be repaired they would sooner or latter get recycled.