Arcadia is the name of a region of Greece known historically for it's mountainous, sparsely settled landscape. The term "Arcadian" eventually became synonymous with any idyllic rural countryside or Garden of Eden-like wilderness.
Dungeons & Dragons
"Where can you taste the joy of obeying unless [God] bids you do something for which His bidding is the only reason?"
- – C.S. Lewis
Arcadia (later expanded into The Peaceable Kingdoms of Arcadia) is the Lawful Neutral (Good) plane of existence where everyone and every thing works towards the common good.
The plane is said to represent the concept of "Greater Good" more so than anything else. Because it's not entirely good aligned, a little bit of evil is tolerated as long as it contributes to the betterment of the majority. That's not to say that things like slavery or genocide are acceptable, but rather the people who make their homes here must give up certain degrees of personal freedom and liberty so shit gets done, Just as Planned without interference or diversions. That also means that despite being generally decent folks, most of the population happen to be a bit paranoid and intolerant of other people who aren't of the same mindsets as their own.
It is the plane most easily identifiable with Lawful Stupid (even more so than its neighbor Mechanus, impressively), as the dogged adherence to law and order of its inhabitants has cost the plane in its past, there used to be three layers of Arcadia, but at some point in its history the third layer, Menausus (or Nemausus, depending on the source), got overfilled by idiots trying to impose RAW so unthinkingly and so unmercifully that the layer just dropped into Mechanus, depending on your edition this was either the fault of the Harmonium bashing too many skulls, or because the Formians migrated outwards.
That hasn't really altered much of the landscape though; as with many other planes each layer is its own infinite space, so (∞x3)-∞=∞, so its not as if there is any less living room. Not only that but because Arcadia is all about regimented order and conformity, each layer looks almost identical, so it's possible that no-one really missed the lost third layer, except the unfortunate people living there when it joined the cogs of Mechanus.
Arcadia is meant to be a utopia where civilization lives in harmony with nature, where rivers and forests run in perfectly straight lines like something out of a 8-bit video game. There are no rolling hills as the landscape is mostly flat, the hills or mountains that do exist as if they had been placed there for aesthetic value more than anything else. The grass grows perfectly even and fruit never falls from the tree but is always ripe for picking.
Day and Night are strictly regimented, there is a "sun" called the Orb of Day and Night that sits itself in the exact center of the plane and visible from all points regardless of your distance from it. It never moves from its spot but it does rotate itself at set intervals so there is no in-between period of dusk or dawn. Just instantaneous light or dark on half of the plane at any given time.
The only thing that can really be said about Abellio is that it it perfectly exemplifies Arcadia, and because there are only two layers for all intents and purposes Abellio is Arcadia.
Militaristic Dwarves make their homes here at Mount Clangeddin, realm of Clangeddin Silverbeard, and spend exactly half of their time drilling or training for war, whether or not the war ever comes since Arcadia is unlikely to come under any real threat. The other half of the time they spend perfecting their craft, therefore it is a really good place to pick up weapons or armour, since outsiders are always welcome (so long as they adhere to the laws). It's also a good place to hire an army if a person were ever so inclined, since the Dwarves here are always tempted by glorious battle and righteous warfare. In the Forgotten Realms cosmology, Mount Clangeddin is connected to Erackinor in Celestia as part of Dwarfhome. Other divine realms include Reorx's Anvil of Creation, Saint Cuthbert's Bastion of Law, Izanagi & Izanami's realm of Cherry Blossom, Marduk's realm of Marduk, Lu Hsing's Ministry of Rewards, and Azul and Tlaloc's realm of Tlalocan.
Formians also live here, having migrated out from Mechanus, but they are quick to reassure outside observers that they are not here to invade or impose their heavy handed brand of order. Whether you believe them or not is another thing entirely.
Buxenus appears almost exactly like Abellio, though the landscape is a bit more rugged and the people are a bit more militaristic and regimented. If Abellio represents a plane of law and order where everyone works for the greater good, then Buxenus is the slightly darker side to that. The second layer represents how law can be harsh and unforgiving, where punishments can be disproportionate to the crime as a means of deterring others. But bear in mind the petitioners and native inhabitants to Buxenus will be of similar mindsets and are extremely unlikely to go breaking any laws, so what we're really talking about is making examples of outsiders and travelers who come to the plane and happen to step on the grass when the sign said not to. In short: don't travel to Buxenus unless you have read the rule-book thoroughly.
The Harmonium make their headquarters here, and are apparently massing for a recovery attempt at the missing third layer so they can bring it back to Arcadia. Though quite hilariously this is doomed to failure, as the only way to move a physical location from plane to plane is to fill it with a majority of people of a particular alignment so that the location itself re-aligns.
While the Harmonium sees that as their goal, what they happen to be blind to the fact that their attempts at indoctrinating people to think as they do has more in common with a lawful viewpoint than a good one, so they run the risk of dropping Buxenus into Mechanus in exactly the same way that the third layer already did. (They aren't called the "Hard-heads" for nothing)
Because of how ordered and harmonious the place is, it's quite easy for people to live here and there are quite a lot of immigrants from other planes, although how they get on depends entirely on their alignment. The Petitioners of the plane (that is, those souls who have died on the material plan and found their way here for their afterlife) get the ability to detect alignment, so they can tell if a creature is lawful/chaotic or good/evil. So Evil or Chaotic beings get caught out very quickly, while those who are neither Lawful or Good are quietly tolerated as long as they can adhere to the laws of the land, otherwise they get asked to leave the plane.
Some of the major powers of the plane are not gods, but are akin to elemental princes or fiendish arch-dukes. The Storm Kings of Rain, Wind, Lightning and Cloud all have their fortresses spaced at equidistant points around the Orb of Day and Night and they control the weather of the plane, making certain that there is full and even coverage.
Changeling: The Dreaming
The mystical, dream-like homeplane of the changelings, mostly lost due to humanity's increasing lack of imagination, wonder and hope.
Changeling: The Lost
Arcadia, probably better known as "one of the many Hells", is the homeland of the mad god-like entities known as the True Fae. It lies beyond the Hedge, a plane in its own right consisting of a wild, overgrown locales covered in thorns that can literally rip your soul apart. Arcadia comes in many different forms, based on the whims of the various Gentry who hold power. They are always horribly nasty places, where humans are spirited away to be forcibly transformed in changelings.
Mage: The Awakening
The Mage version of Arcadia is a dreamy, fairytale realm that hosts the Watchtower of the Lunargent Thorn. Mages who Awaken by signing their names there become Acanthus, Enchanters on the Path of Thistle. Its dominant Arcana are Fate and Time, whilst its inferior Arcana is Forces. Mages have no idea why their Arcadia and the Lost's Arcadia is so different; experiments to go there never go well, since getting to Fae!Arcadia means going through the Hedge... which rips at human souls... which Mages need to use their magic. Yeah, it doesn't work out very well for anyone, and the Changelings wish the mages would Stop. Fucking. TRYING!