[derefr]

And the etymology of this being that Dungeons and Dragons has a "Prime Material Plane" and an "Astral Plane", where the Astral Plane connects the PMP to various "Outer Planes" made of ridiculous not-oft-encountered stuff.

But whoever came up with this cute analogy got the analogy wrong — the higher Unicode planes are analogous to the "outer planes" themselves; while the "astral plane" would be some sort of glue allowing you to access these outer planes from within the BMP. Like... surrogate-pair characters! One could nickname the reserved surrogate-pair range in the BMP, the "astral projection" range ;)

[kens]

"Astral plane" predates Dungeons and Dragons by centuries. Looking at old discussions, I couldn't find any evidence that Unicode's usage is connected with D&D.

Early discussion of "astral character" or "astral plane" for the Unicode supplementary planes at: https://unicode.org/mail-arch/unicode-ml/Archives-Old/UML024... Even earlier 1998 use: https://www.unicode.org/L2/L1998/98354.pdf

[Sniffnoy]

The term "astral plane" is older than D&D, and I would assume they took it from the more general usage, not the specific usage in D&D. https://en.wikipedia.org/wiki/Astral_plane

[GauntletWizard]

I’ve met several of the Unicode standard committee - They’re nerds. The kind of nerds for whom “Astral Plane” is a multilayered joke. It’s not not about the general usage, but nor is it not about the D&D term.