Aether refers to the energy found throughout the world of Hydaelyn in Final Fantasy XIV. It is source of both magic and life, and any disruption in the aetherial flow is either the cause or indicator of catastrophic events, as evidenced by the Umbral Eras.
Aether is released from the body upon death and then disappears from the physical realm. Only so much of it can cross over at a time, and any excess remains to form a glowing mist, crystals, or even ghostly apparitions. As aether is akin to soul energy, more violent and dramatic deaths tend to yield more excess aether.
When a living entity arrives on the scene of a battle, no matter how ancient, the excess aether will enter this new host and expand its native aether. This generally augments the creature (human or otherwise) physically, and those unaccustomed to channeling the new energy may go mad.
Aether flows nigh imperceptibly throughout the planet, and places where it collects most are typically vivid, particularly in the Black Shroud. The Black Shroud is so rich in aether that the sentient, elementally aspected aether-beings known as Elementals reign over the wood, including the cooperative Gridanians.
Aether is considered dangerous in high concentrations, and often causes the afflicted to become more aggressive, if not completely maddened. Monsters in aether-abundant areas are more threatening.
The primals are thus surrounded by a disproportionately high concentration of aether, and adventurers are not allowed to stay in their lairs for too long (though this detail has not been mentioned since 1.0).
On a long term scale, primals will consume aether from their surroundings, and if left unchecked, would drain the land of all aether.
Much like the lairs of primals, instanced dungeons, such as Thousand Maws of Toto-Rak and Dzemael Darkhold, have dangerous aetheric levels. Adventurers will be forcibly teleported to safety after a set amount of time.
In some cases, overexposure of aspected aether causes profound mutations in living beings. The Sahagin and their primal Leviathan sometimes punish errant Serpent Reavers by exposing them to over-aspected water aether until they mutate, resembling marine life.
When an adventurer reaches an area's Aetheryte Crystal or Gate they align themselves permanently to the aether of that area, allowing them to teleport there usually at the expense of Anima. There are also nodes that teleport adventurers to various instances and battles, nodes that appear at the end of a Guildleve to return to camp, and "Aethernet" crystals for teleporting across town. After the dramatic aetheric shifts in the Calamity, the redesigned Aetherytes no longer require Anima, though the loss of Anima use may have led to the inability to use natural Aetherial Gates.
The Beast Tribes have devised their own crude Aethernet networks, and adventurers can use these to reach the lairs of primals.
Small aetherial crystals are used by Disciples of the Hand for Synthesis. Their largest forms are clusters, but single crystals and shards are used for most recipes. Each crystal has an elemental aspect, such as Fire or Wind, and can go unstable during synthesis.
There are also "Deaspected Crystals" and "Over-aspected Crystals" for specific purposes. They have either no aspect or an unusually strong aspect. Deaspected are gray (no elemental color) and over-aspected are orange, a trait often attributed to corruption.
Ceruleum is a neon blue substance and a form of aether refined in reactors. It is a volatile energy source used predominately by the Garlean technology, much of which was borrowed for Eorzean use in things such as airships and smaller powered devices.
If even a spark comes in contact with a crate of Ceruleum, it could cause region-wide devastation akin to a nuclear blast.
Aether is found as a weather effect known as Gloom in Mor Dhona, and it appears as a psychedelic purple miasma. In addition to limiting visibility and altering the effectiveness of certain spells and abilities, this weather condition used to cause the Notorious Monster Dodore to spawn.
As mentioned by Cid Garlond, the flow of aether has changed. This was one of the first signs of the impending Seventh Umbral Era. The Garlean Empire used vast amounts of aether to bring Dalamud down from the sky causing the shift of aether surrounding Silvertear Falls.
Although an aether sickness is now present, for some reason the dangers of overexposure to primal levels of aether are no longer mentioned.
The yet unexplained presence of Atomos drained the Aetheryte Crystals of their energy corrupting them and turning them orange enabling free teleportation without the use of Anima. Eorzea as a whole has harnessed this and now teleporting costs gil for upkeep of the Aetherytes (a fee collected by the gate guards in each camp). New Aetheryte Crystals were constructed to replace those that were lost.
Aether exists beyond the planet, and can be harnessed. The Allagan civilization used both the Crystal Tower and Dalamud to collect and store aether from sunlight. The practices of Sharlayan Astrology can harness aether from the constellations.
- The aetheric properties of Mor Dhona can cause synthesis crystals to lose their aspect if left in the ground for too long, but this concept only appeared in one quest in 1.0. There were also Deaspected Crystals found through Seventh Umbral Era quests, which could be redeemed for rare prizes.
- Aether is part of a series wide trend to use a supernatural substance to explain the origins of magic, crystals, life, and other phenomena. Other instances of this include Mist, Miasma, and the Lifestream/Mako. Of these, aether is conceptually closest to the Lifestream: both occur naturally in multiple degrees of density and both are poisonous at extreme levels.