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 socioeconomic advancement of the setting, and many lame-brained "advances" in story from edition to edition, most infamously with 4th edition's "Spellplague" and retconned twin planet where all the new 4e races were hiding.
Note that in high-magic settings, sorcery sometimes gets so common and overpowered that it basically replaces technological progress. Why would you build robots or rockets if you can just create golems or cast Teleport Without Error?
Another issue with medieval stasis is that a lot of writers, most of them in fact; probably know less about the actual middle ages than the average Crusader Kings 2 player and thus present not only a world in medieval stasis but one that's in at best, a theme-park version of the medieval period and quite often only really showing Anglo-French medievalism (and a bastardized shitfarmer version of it at that). The somewhat more historically literate might put in some anachronisms like references to ancient Greece, Egypt, and Rome or to the Aztecs (usually a ramshackle mishmash of half remembered tidbits of the Mayans, Aztecs, and Inca thrown together with no real thought) and if you're extra lucky you might get something that's an extended reference to a (largely inaccurate) medieval Islamic polity or to the Holy Roman Empire mixed in with the usual barbarian tribes but that's usually about it. Like the Democracy thing mentioned above? It was nowhere near that simple in real life. A great many of the tribal societies we have records of were actually very democratic, where the King was elected and so were the chiefs below them and they absolutely did not have absolute authority over their subjects. And of course "feudalism" is simply a catch all label for a hugely varied and complicated array of societal organization systems that can be vaguely described as an aristocratic hierarchy based around land and military service and assorted ties of loyalty and bloodline.
And even in medieval Europe you had systems that broke the norm, like the Merchant republics of Italy or the north German Free cities and of course you had lands directly ruled by the Church. Never mind that you also had rather different systems of organization elsewhere in the world like in the Islamic world, India, the Americas, and of course China's quite literal bureaucracy where civil servants hired based on their performance in examinations did most of the day to day governing of China; dynasties could come and go but the bureaucracy was eternal. Tolkien was himself of course, a medievalist with very deep knowledge of the time period even by today's standards with our rather improved access to knowledge of the time period. Warhammer was created by history nerds who very much knew what they were writing about and so populated the world of Warhammer Fantasy with references to just about every political system that predominated in the medieval and renaissance periods as well as a lot of those that predominated in antiquity. So not only does Medieval Stasis perpetuate an annoying degree of sameness in the fantasy genre, it also tends to be based on a conception of medieval times that's not only essentially completely limited to France+England with some scattered references to other stuff, but is also almost completely wrong about everything and doesn't even scratch the surface of the depth of medieval history.
Notable Examples of Medieval Stasis
- 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.
- 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: Westeros is extra static, 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) which is something patently absurd when you consider how much real life royal, imperial, and noble families have had to struggle to avoid patrilineal extinction in just a few centuries, with the oldest still extant aristocratic house being the Japanese house of Yamato and even then it's likely that they bent the rules of succession at least once in their 2500 year history. 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. At least until 4th edition blew it up along with everything else fun or interesting in the Forgotton Realms. As of 5th edition, the current (albeit scattered and/or vague) lore seems to imply that Lantan's destruction has been retconned like the rest of the Spellplague.
- 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: Bretonnia is literally in Medieval Stasis despite having one of the most technologically-advanced nations right next door. The Elves of all types give no fucks about advancing their technology, but in their defense 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 Settings 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, but the problem is that they are under constant Chaos invasions and Chaos Gods themselves are not above screwing with the world, which puts something of a crimp on pure research. Imagine what Nurgle would do to the guy who discovered penicillin in this world. The fact that relations 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 users 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) and it's almost all magitech based on weaponizing solidified Chaos. 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 isn'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 fantasy. 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 weird and expressly focused on subverting the usual D&D cliches, so the technology is a strange mixture of all eras with a side order of JRPG-style magitech. 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. 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. Not bad for a furry RPG, huh?
- 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: Golarion features 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, the Chelaxian breakaways Andoran and Galt have started to push for a less aristocratic 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 steampunk robots to fight the Dragons and the whole world is now in a full on steam-powered Industrial Revolution without the gunpowder.