It's a term from gaming which, in the broadest sense, means how they paint the sky to make it look authentic. Imagine being inside a cardboard box - the sides are painted to look like rolling mountains and hills, and the top face is painted to look like a cloudy sky, and if it's done well, all of these should connect up and give the impression that you're lookint at a real sky.

[–] [email protected] 10 points 1 year ago* (last edited 1 year ago)

Yes, and depending on the engine, the skybox can even be spherical and/or “infinite”. Without a skybox, many engines will draw geometry over the previous frame, resulting in a dizzying non-refreshing background (that often gets psychedelic due to repeated color effects).
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