Ah, well, that’s a dictionary definition reflecting lay usage of the term (i.e. how people use it in novels and the like), but it’s not the clinical definition. Delirium is a specific medical syndrome (https://en.m.wikipedia.org/wiki/Delirium).

The simplest way to explain it is a double definition-by-contrast:

1. You’ve got https://en.m.wikipedia.org/wiki/Locked-in_syndrome, where people have thoughts and intent but (mostly) can’t translate them to motor commands. But these people are clearly trying to do things, as evidenced by the fact that they can often (learn to) move their eyes to communicate what they want.

2. You’ve got https://en.m.wikipedia.org/wiki/Akinetic_mutism, which has similar symptoms to locked-in syndrome, but is due to a lack of will to move rather than a lack of ability. (Or—since that sounds a bit like a free-will thing—they have a lack of ability to induce the cognition required to conclude that movement is desirable.)

3. Then you’ve got delirium; delirium is a syndrome [cluster of symptoms] like akinetic mutism, and both have nearly identical symptoms in acute presentation—but very different symptoms when only in partial presentation, which is why they’re not the same syndrome. The syndromes are somewhat hard to tell apart, but once you’ve figured out which one someone has, it’s usefully diagnostic, leading you to a different etiology (i.e. helping you tell what the root of the problem is.)

The syndrome of akinetic mutism is pretty much only caused by specific physical brain damage (like a stroke.) You can think of it as some part of the brain necessary for the “thinking” and “wanting” process, just dying, breaking that mechanism irreversibly. Usually this is damage to a pretty large region of brain tissue, and may indeed involve loss of experience of conscious qualia as well. (It’s hard to tell, since an akinetic mute is hard to interview.) But they’re not in a https://en.m.wikipedia.org/wiki/Persistent_vegetative_state, since they do autonomic things, often enough to keep themselves alive (but only barely.)

Delirium, meanwhile, isn’t caused by physical brain damage, but rather by some other organic (physiological) process that results in electrochemical changes to the brain which then result in a similar symptom profile as akinetic mutism. This is often reversible and doesn’t actually involve the death of any part of the brain, which is why it’s helpful to think of this as just a “state of mind” that is next-to-impossible to achieve without weird chemistry happening in the brain—sort of like the states of mind found under the influence of psychedelics.

But—and this is just me going out on a limb—I have a feeling that delirium is one of the many altered states of consciousness that can be achieved through meditation (since it can also be achieved by letting oneself sink into the depths of major depression, and that sinking-into is—in theory—an entirely psychologically-driven neurochemical process, just like meditation is.) And, if it’s possible for a regular, healthy human being to put themselves in an acute delirious mental state (recall, this matching the symptom profile of akinetic mutism—a complete lack of desire) then I do believe that this is an entirely sensible thing to “prescribe” to people as a technique for avoiding earthly suffering, if one is trapped in it (e.g. if one is a prisoner of war being tortured—as the samsara conceptualization of naraka is likely a metaphor for; or if one is born into famine, poverty, and strife, never knowing any goal beyond animal survival—as the preta realm is likely a metaphor for.)

In other words, delirium is a—perhaps healthier—alternative to entering a dissociative fugue state, as a coping strategy for intense trauma. Delirium involves neither depersonalization (feeling detached from yourself) nor derealization (feeling detached from reality), but instead just involves detaching oneself from one’s desires. “You” are still there, and “reality” is still there; you are mindful, there, in the moment. The only thing that is gone is the agent that was dissatisfied with the present relationship between you and your reality.

I think I see what you're saying. I wonder what you think about the descriptions of the deepening levels of dhyana which do involve "detaching from reality" in some sense. Even the one-pointed concentration needed to approach the first dhyana stage involves tuning out almost all normal perceptions, for example by sitting in a candle-lit room and concentrating fully on the breath in the nostrils. But you keep a strong and stable kind of agency in order to maintain your concentration and to progress in the dhyana series. It seems different from being a "qualia sponge" with a delirious lack of desire or intent. The Wikipedia page for clinical delirium describes distractibility and failure of attention as a primary symptom, whereas deep meditation is almost always described as a result of non-distractability and highly trained attention...