Some settingshavePhlebotinum. It helps move along the plot, Hand Wave various characters' improbable powers, and conveniently get the Heroesfrom point A to point B quickly. It can let you just say A Wizard Did It.
But then some settings...are different. They have nothing but Phlebotinum. That City of Adventure the heroes are exploring around? It's completely floating in the air; not one building is touching the ground. Why? Phlebotinum! In fact, the whole planet is probably bound together with some sort of magical or super-science energy, without which it would simply explode. Every single thing around requires whatever the local flavor of Phlebotinum is to run in worlds such as these, whether it's magic, nanomachines, The Force, or something else similar.
The main dividing line between a world that simply has a lot of Phlebotinum and one that's made of the stuff is this litmus test: if you took away the Phlebotinum, would your world still exist in any meaningful sense, or would it more or less just collapse? If it would, your world is just plain Made Of Phlebotinum.
There is actually a great deal of evidence that Real Life is thus: when you develop a potent capability to the point that there are few side effects, you start using it for everything imaginable. When we learned to harness electricity easily, the only things weren't 100% electrical were the things that we used to generate electricity. When computers became advanced enough, we started carrying them around with us. Litmus Test? If you took away computers, it'd re-create The Great Depression. If you took away electricity, ninety percent of the world would die within a decade.
If something is literally made of Phlebotinum, see Made of Magic.
Youtube Commenter: I'm not sure which is weirder, the way the universe in this series works, or the fact that the universe works EXACTLY HOW KAMINA THINKS IT DOES!
Case in point: Kamina tries the whole Combining Mecha trope by literally smashing together the two parts of the Gurren Lagann. It actually works. Sort of.
In David Wingrove's Chung Kuo, the world-spanning City is made of "Ice"
Discworld can seem ordinary enough at first glance, until it's pointed out that, without heavy duty magic involved, a flat world on the back of a giant turtle that swims through space should be utterly impossible.
The Science of Discworld books go on to give the Phlebotinum a name: narrativium, which (together with chelonium and elephantigen) in fact makes the formation of flat worlds on board giant turtles and elephants not only natural, but inevitable. Too bad Roundworld lacks these perfectly normal elements and had to concoct some outlandish alternative involving big rocky balls...
In The Last Hero Discworld comic, Cohen and his Silver Horde have made it their mission to bring fire back to the gods —- via a large explosive. This would suddenly remove magic from the world and cause everything to die.
The Roman-based society of Codex Alera is so connected with their Phlebotinum of Furycrafting that the use of certain techniques and technologies, such as quarrying, are a lost art.
The Chronicles of Thomas Covenant. Then, in the Second Trilogy, They lose the phlebotinum. This is way more horrific than it sounds. In part, because they eventually realize that the phlebotinum hasn't actually been lost: it's become corrupted and is causing the various disruptions that the world is experiencing (rainstorms, droughts, pestilences etc.)
Sixth Column by Robert A. Heinlein is heading toward this in the end. The harnessing of magneto-gravitic, electro-gravitic and ternary fields promises to change the world even more than electromagnetism did and much faster. Everything The Resistance uses, both weaponized and mundane is based on those new discoveries.
Ravenloft literally so - it's a series of artifically-created "demiplanes" floating in the misty emptiness of the Ethereal Plane. When a domain's Cosmic Keystone is destroyed, it may be absorbed by neighboring domains, or it may simply collapse into the Mists.
Exalted, full stop. Creation itself is the greatest artifact ever built, while Odd Job Gods exist for individual rice grains and their interactions provide the physics of the universe.
The Phlebotinum is named Essence, and it is the essence of everything ever. Even the unshaped raksha are made of it and use it to fuel their eldritch powers. Its name is used in many places where we would use the word "atom".
According to this video, Azeroth is a sphere about 12 kilometers across, with a density around a hundred times that of Earth. This explains why when you drop an item it disappears; due to the gravity it is crushed into an extremely fine powder spread over a wide area.
In Chrono Trigger, the magical floating continent of Zeal is this. It completely depends on magical energy to stay aloft and its residents do pretty much everything by magic.
Tales of Symphonia and Tales of Phantasia are like this. The main focus for most of both games is preserving the mana flow and thus preventing the collapse of the world. Without it, magic won't work, crops won't grow, etc.
The consistency of this trope throughout the series is one of the reasons the Synthesis ending of Mass Effect 3 was considered so terrible. It had nothing to do with the mass effect fields. Nor was it generated by element zero.
In the blog-novel Flyover City! – Malphysical Particles create a cosmic loophole which render the laws of physics “malleable”, allowing the story’s superheroes to do all those superhero-y things that are impossible in the real world.
"Physical laws? Oh, those things. We don't have them." "But everyone has them. It's a fundamental principle." "Not in this world. Everything is taken care of by magic in various forms. What's holding you to your seat isn't gravity, for example." "Oh, what then?" "Magic. Don't look so alarmed. It's at least as reliable as gravity is."
The four fundamental forces of the universe. We know how they work; the why on the other hand would need a theory of everything (Or at least this universe).
The fact that everything is made out of matter is a bit of a headscratcher in itself. Matter and antimatter must have been made in equal quantities in the big bang, so why didn't they just annihilate each other?
Life. Its all over the place, but we don't really have any concrete ideas about where it came from or how it started, and haven't had much luck finding it anywhere else so far.
Money. (Which may count as fundamental force of the universe as its been stated to make the world go round.) Its been around in some form or another since the Stone Age and if it were to suddenly vanish, Human Civilization as we know it would likely go up in flames. (The damage getting worse the longer money is gone.)
First, it just cannot "suddenly vanish". Second, money have no value on its own - it's only a medium used to exchange fruits of labor and someone else's labor. You cannot buy anything if there's simply nothing around that could be bought and if money somehow disapper (or, more likely, become worthless), the goods, the food and the houses won't disapper with them. Third, in how many Utopias there is no money at all?
It could be argued that a medium of exchange is an emergent property of the human mind... barter appears to have been ubiqitous, and many quite diverse cultures invented money separately.