Dath Ilan is a fictional world and civilization invented by Eliezer Yudkowsky. It is a parallel reality of earth where society is much better at coordination and various good policies have been implemented, though technology is only as advanced as earth's, if not slightly less. It can be thought as a more practical and realistic form of Economist's Paradise. Eliezer first introduced it in his April Fool's day post 'My April Fools Day Confession', where he claimed that he was an average person from that world and none of his ideas were original.

This world was further fleshed out (and some its backstory changed) in a later April Fool's post:

I'm from a parallel Earth with much higher coordination: AMA

And in a series of glowfics featuring dath ilani characters:

a dath ilani matchmaker in King Randale's court (incomplete)
but hurting people is wrong (complete)
mad investor chaos and the woman of asmodeus (ongoing as of Apr 2022)
a dath ilani EMT in queen abrogail's court (ongoing as of Apr 2022)

Some known features of dath ilan:

Standard education includes rationality training (from which, allegedly, all of Eliezer's own ideas are plagiarized).
What this Earth knows as "logical decision theory" / "Functional Decision Theory" is just standard decision theory as formulated in dath ilan.
Land Value Tax, positional-goods tax, status-goods tax, marketing-tax, no income tax.
Movable Homes.
Autonomous electric cars in tunnels instead of ICE cars on roads.
No streetlights at night, except for red lights along walkways, which for 45 minutes each night also turn off to see the sky. And on winter solstice (night of stars) the lights stay off the whole night, to give a perfect view of the night sky.
The average IQ in Earth-equivalent terms is 143, after an unknown number of generations of "heritage optimization" pursued via positive government subsidies.
Dath ilan has a lower population than Earth (1 billion people), since their higher intelligence and higher coordination enabled them to make tech progress without having first attained a higher population than that.
There exists a single global civilization, calling itself "Civilization", with one world government, called "Governance".
All humans (and other hominids, like chimpanzees) are cryopreserved upon death.
If you commit a murder but don't want to consign your victim to true death, you can call in a government service, the Surreptitious Head Removers, who will remove your victim's head for cryonic preservation, while being sworn not to report on this.
Dath ilani holidays are known to include the annual Alien Invasion Rehearsal Festival and the annual Oops It's Time To Overthrow the Government Festival.
There exists a group called Keepers who undergo much more rigorous rationality training and act as a counterbalance to Governance; the dath ilani equivalent of Leo Szilard eventually told Governance about her idea for nuclear weapons, but she talked to the Keepers first.
In the fictional setting (though this concept didn't appear in the original April Fools post) dath ilan has found it necessary to try to causally screen off its entire history from its present - all old books hidden away, all old cities mothballed. As the Doylistic result, dath ilani characters ending up in settings with much lower social technology levels, such as Valdemar, Golarion, or Earth, start with little prior concept of how a less-coordinated society looks, and must learn from experience.
dath ilan doesn't have earth-like banks, instead people invest in equities, which automatically get sold off in fractions whenever they buy something.

Not tagged due to spoilers:

Lies Told To Children

External Posts:

Until we Build dath ilan