firmament

UK: /ˈfɜːməmənt/ US: /ˈfɝməmənt/ ,(fûr′mə mənt)

WordReference Random House Learner's Dictionary of American English © 2025

fir•ma•ment /ˈfɜrməmənt/   n. [countable* usually singular: the + ~]

1. the arch of heaven;
   sky.

WordReference Random House Unabridged Dictionary of American English © 2025

fir•ma•ment  (fûr′mə mənt), n. 

1. the vault of heaven; sky.

• Late Latin firmāmentum sky, Latin: support, prop, stay, equivalent. to firmā(re) to strengthen, support (see firm²) + -mentum -ment
• Middle English 1250–1300

fir•ma•men•tal  (fûr′mə men′tl), adj. 

Collins Concise English Dictionary © HarperCollins Publishers::

firmament / ˈfɜːməmənt/ noun

1. the expanse of the sky; heavens

Etymology: 13th Century: from Late Latin firmāmentum sky (considered as fixed above the earth), from Latin: prop, support, from firmāre to make firm¹

firmamental / ˌfɜːməˈmɛnt^əl/ adjective