Definitions from Wiktionary (firmament)

▸ noun:  (usually uncountable, literary, poetic, also figuratively) The vault of the heavens, where the clouds, sun, moon, and stars can be seen; the heavens, the sky.
▸ noun:  (countable) The field or sphere of an activity or interest.
▸ noun:  (uncountable, astronomy, historical) In the geocentric Ptolemaic system, the eighth celestial sphere which carried the fixed stars; (countable, by extension) any celestial sphere.
▸ noun:  (uncountable, obsolete except biblical) The abode of God and the angels; heaven.
▸ noun:  (countable, obsolete) A piece of jewellery worn in a headdress with numerous gems resembling stars in the sky.
▸ noun:  (countable, obsolete, also figuratively) A basis or foundation; a support.
▸ noun:  (countable, obsolete) The act or process of making firm or strengthening.

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