English from the 13th century. From Latin firmāmentum (from firmō (“strengthen”), from firmus (“firm”)), literally "that which strengthens or supports". The term is coined in the Vulgata in imitation of LXX στερέωμα (“firm or solid structure”), which in turn translates Hebrew רקיע, strictly speaking a mistranslation, as the original Hebrew term meant "expanse", from the root רקע "to spread out", which in Syriac had acquired the meaning "to make firm or solid".
firmament (plural firmaments)
- (uncountable) The vault of the heavens; the sky.
- And God said, "Let there be a firmament in the midst of the waters, and let it separate the waters from the waters."
- (obsolete) basis.
- The field or sphere of an interest or activity.
- the international fashion firmament
- (archaic) In the geocentric Ptolemaic system, the eighth sphere, which carried the fixed stars.
Derived terms 
- Oxford English Dictionary, Second Edition, 1989