Medieval Stasis describes the state of essentially all fantasy worlds that never get to steampunk.
As the title implies, most fantasy worlds are stuck at a technological level roughly equivalent to Europe between 1000 and 1500, being more advanced in some fields and more primitive in others, until the universe collapses. A knight's ancestors five thousand years ago fought against Orcs on the back of a great warhorse, wielding sword and lance, wearing plate and a greathelm, just as he does at present. At best, some groups in the universe may be more advanced than others (some peoples might be building castles and forging plate armor while others live as primitive cave men armed with flint axes and stone tipped spears), but nobody will be developing new technology, or, on the off chance one or two factions are, it will never spread much or catch on anywhere else. This also applies to social structures such as feudalism, with a max of one non-Greco-Roman democracy per setting. It will be conquered and restored from edition to edition as fanboys war behind the scenes.
While it is not, in and of itself, a bad thing, as it creates a set mood and style of play, we run into the fact that many writers are hacks, and use it to both rip-off other writers (principally, Tolkien) and to keep the world stagnant enough that they don't risk smashing something people actually like that they didn't have the skill to realize they shouldn't smash, while still maintaining the illusion of forward momentum. The Forgotten Realms is a prime example of this, featuring both several powerful organizations out to stifle any attempt to progress the technology or socio-economic advancement of the setting, and many lame-brained "advances" in story from edition to edition, most infamously with the "Spellplague" of 4e.
Important thing to note is that in some of those settings magic gets so common and overpowered that it basically replaces technological progress. Why would you build robots or rockets if you can create golems and just open portal straight to the moon?
Notable Examples of Medieval Stasis
Remember: Medieval Stasis is the norm of most fantasy worlds, so don't list everything under the sun or we will be here all day, only leave the most egregious or well known offenders
- Lord of the Rings: Tolkien was a naturalist who wasn't too fond of industrialization, having seen the First World War's highly industrialized warfare up close and personal, so the heroes of his stories preferred Medieval Stasis as well, barring a few anachronisms like clocks and matches. Unlike most of the writers that he inspired, Tolkien had five hundred pages of background explaining why, namely because Middle-earth was in a state of decline due to the ravages of Morgoth and Sauron, the gradual decline of the elves and the Dunedain after the downfall of Numenor, and much of their technology was given to them by the Valar rather than inventing it themselves, so it's much more excusable.
- The funny thing is, based on supplementary books and scrapped stories, Numenor came quite close to being a Steampunk world power equipped with steamships and even rockets.
- A Song of Ice and Fire: A really notable example, because not only has everything been fairly stable for thousands of years until the Great Fuckening of the current time frame, some individual families have had unbroken rule over their lands for a hundred odd generations (The Starks being the prime example, as they have ruled in Winterfell for over eight thousand years.) That said, it should be noted that part of the backstory involves the Bronze Age First Men defeating the Stone Age Children of the Forest, who were themselves conquered by the Iron Age Andal invaders everywhere but in the Iron Islands and the North (who adapted and adopted the technology of their would be conquerers), and the records of the ancient days are spotty at best, full of mythical accounts and many of the Maesters believe that said events happened over a shorter timeframe.
- Forgotten Realms: Not only have things been more-or-less exactly the same for all of recorded history, there is a powerful, international, theoretically-good-or-at-least-neutral organization actively devoted to making sure that no progress of any kind is ever made: the Harpers. Whenever anyone invents something useful (guns, locomotion, etc.) and tries to market it, the Harpers confiscate it. Whenever a good-aligned king tries to unite and stabilize the warring states, the Harpers murder his ass. Faerun hasn't budged an inch since Ao glued it together. The only exception to this was the island nation of Lantan. The island was a theocratic state in service to Gond Wonderbringer, a deity whose portfolio included innovation and technology, who gifted his followers with knowledge of smokepowder which lead to functional in-setting firearms. However, 4th edition played this trope straight by annihilating the island. As of 5th edition, the current (albeit scattered and/or vague) lore seems to imply that Lantan has returned in some capacity.
- Greyhawk: Despite the fact that the current page on this oldest-of-the-old school settings is little more than impotent bitching at the moment, it also has a society where nothing much ever has happened or will happen to bring about changes in the lifestyles of its inhabitants. And this is the setting with a literal god of Old West gunfighting and an army of firearm-toting paladins analogous to sheriffs.
- Dragonlance: Apocalyptic calamities come and go, but Krynn stays at pretty much the same level of pseudo-medieval tech forever, world without end, amen. And, no the tinker gnomes do not count, since their stuff almost never does anything useful, gets mass-produced, or catches on outside the gnomes themselves. In fact, some material explicitly says that the reason for the stasis is because of the fucking gnomes; their absolute idiocy when it comes to producing technology has actually convinced pretty much every other culture on the planet that science is fundamentally inferior in every way to sorcery! The one culture that doesn't think they're entirely a waste of time is only interested because it pretty much hates magic... and is made of a bunch of knight-in-shining-armor types so hidebound that they haven't been able to properly fix their organisation since the first Cataclysm, and so anything like vehicles or gunpowder is certain to get dismissed on grounds of being "dishonorable". So, yeah, fuck tinker gnomes.
- Warcraft: In a cartoony match for the Dragonlance example above, Azeroth's many factions never adopt one another's technological advancements. Goblins and gnomes can invent as many steampunk robots as they want, none of their stuff will ever change the world in a concrete way. Even the aliens are mostly just sword-and-sorcery types. That said, firearms had established themselves in the comparatively recent past.
- Ravenloft: This is probably the most interesting example. The Demiplane of Dread doesn't so much "advance" as it does "absorb some place where things are a little more complicated," and most of the Domains of Dread are already tailor-made just to torture their prisoners. Thus, though individual Domains might be advanced enough for common people to have firearms and gaslights or so primitive that they aren't even into the Stone Age (King Crocodile for the win!), they will almost never learn from or assimilate one another's technology. Each Domain will be mostly frozen into the level it's at, medieval or not. Amusingly, this works both ways: technologically-advanced societies are no more likely to take up magic than lower-tech ones are to learn to use gunpowder.
- Warhammer Fantasy Battles: While the Empire, Dwarfs, and to a lesser extent the Skaven all have technology, some factions in Warhammer play this trope straight. Bretonnia is literally in Medieval Stasis despite having one of the most technologically-advanced nations right next door. The Elves (all types); In their defense they may not have changed in thousands of years but what they have still works, they have access to giant monsters such as dragons and hydras and the Dark Elves at least have progressed from bows to rapid-fire armor-piercing crossbows. The Warriors of Chaos are again literally medieval, but in their case they're Medieval Vikings. Orcs have not been introduced to the wonders of "Dakka" yet; the Lizardmen still use wood and stone but make up for it by also using dinosaurs and advanced magic. Lastly, the Ogres are pretty much in "Stone Age Stasis" as they're not very intelligent but under Overtyrant Greasus started to discover the benefits of commerce.
Notable Examples without Medieval Stasis
- Warhammer Fantasy Battles: The Empire and the Dwarfs are actually about the level of most European countries around 1500, at the start of the early modern period and the Renaissance. They're also advancing, albeit slowly, the problem is that they are under constant Chaos invasions, which don't help, and that the chaos gods are not above screwing with the world. Imagine what Nurgle would do to the guy who discovered Penicillin in this world. The fact that the relation between the engineers and the cult of sigmar are not the best in the world does not help things at all. The other notable technology user are the Skaven, but the Skaven technology only effects their weapons (god help the world if they ever figure out sanitation considering what it did to our own population) further a case can be made that they don't use technology, but build machines that use magic given how much a role Warpstone plays in their technology. On the other hand: there is little practical difference between the two. Undead straddle the line between the two, with the vampires not being afraid to use technology; the problem is most of their undead minions lack the physical and mental acumen to use it while the vampires physical, mental and magical abilities make technology practically redundant to them at a personal level. The tomb kings had technology at the steampunk level, though this wasn't represented in the game, but they are more concerned about rebuilding their realm, which has fallen into disrepair due to hundreds of years of civil war and no maintenance, rather than advancing their society. They do have something like robots in the form of their magically animated undead constructs.
- Iron Kingdoms: The Iron Kingdoms setting is one of the best examples of steampunk. They're developed to the extent of the Victorian era (the mid-to-late 1800s), with a slow-but-growing industrial revolution and the discovery and development of electricity and chemistry. At the same time, it remains a recognizably fantasy setting in many ways, with wizard orders, barbarian tribes, and dangerous monster threats on the frontier demanding plucky-adventurer solutions.
- Eberron: Eberron is so-called "dungeon punk," so the technology is a strange mixture of all eras (plus a lot of magic!). It's one of the few settings that avoids both medieval stasis and outright steampunk, since magic is so common that it has effectively displaced technology in-setting. And because there is no continuity and every game starts at exactly the same point in time as every other game, there's no real status-quo to worry about upsetting.
- Dark Sun: A weird example. Depending on edition, the past of Athas may have included anything from a standard fantasy setting to a bio-mechanical halfling empire. But, either way, the Brown Age is a barbaric decline of these past glories, with little metal and no feasible way of shaping more leaving the world in an oddly-civilized neigh-Stone Age. Still, there is an undercurrent of rebuilding and reforming throughout the more-heroic-minded books on the setting, helped by the same eventual anti-continuity Eberron had, so the idea that things could progress or get better isn't impossible.
- Ironclaw: The once-fantasy world is undergoing a pseudo-Renaissance shift away from magic and feudalism to machinery and Italian-style guild-republics. PCs are actually explicitly part of the burgeoning new middle class.
- Mystara: Depending on where you are, there might be airships, magic-powered technological conveniences, and drill-tanks to explore the hollow earth full of dinosaurs. Either way, things are a little less generic here in proto-Eberron.
- Pathfinder: The default Golarion setting includes relatively advanced technologies such as flintlock and matchlock firearms, the printing press, galleons (crewed by pirates reminiscent of the Golden Age of piracy in the Caribbean), and, in certain sourcebooks, steampunk/magi-tech spaceships. Not to mention the number of people whose clothes and equipment are explicitly based on 18th-century fashions (see, among others, Andoran, Taldor, and Alkenstar). Also, there's that one random corner of the world where aliens are trying to peacefully settle and/or invade, only to realize they picked the *one* corner of the world where pleas of "We come in peace!" are met with warcries and the judicious application of battleaxes to various vital areas. A recent sourcebook includes *lots* of super-high-tech stuff and different class archetypes that make use of it. On the socio-political front, places like Andoran and Galt have started to push for a less feudal aristocracy-type government.
- Avatar: The Last Airbender: It was true in the past, but by the time of the original series the Fire Nation has become an industrial power, complete with colonial ambitions towards the rest of the world. In fact, the main character's previous incarnation as Avatar actually stopped the Fire Nation from breaking medieval stasis because he foresaw that doing so would mean allowing them to subjugate all the other peoples. At the end of the show, the protagonist makes peace between all three surviving factions, and the sequel reveals that doing so helped the world advance to a roughly 20s/30s era of technology, complete with automobiles, moving pictures, professional sports, politically radical revolutionaries, and cronyist democracy.
- Dragonmech: Dragonmech's setting used to be in Medieval Stasis, then chunks of the moon started to rain down on them along with Alien Moon Dragons riding the rocks down for a full-on invasion, people first hide underground but then a dwarf kick starts the creation of Pacific Rim sized steam power robots to fight the Dragons and the whole world is now in a full on steam power Industrial revolution without the gun powder.