The Firmament (Also known as the Welkin) is the line that separates different levels of the sky or universe in Judeo-Christian and Mesopotamian mythology. It’s mainly (but unproven) used for the celestial dome that separates the cosmos from the Heavenly Chambers. Originally, it was the vault of the Universe in older forms of astronomy. However, under the influence of Abrahamic religions and the Sumerian Garden, as well as the evolution of English, it was reinterpreted as the vault of Heaven. In canonical Islamic sources, the firmament is still considered to be the vault of the Universe while the throne is seen as the vault of Heaven/Illyin/Jannah.
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https://quod.lib.umich.edu/m/middle-english-dictionary/dictionary/MED20700