The sky conceived as a vast dome or vault overhead, once imagined as a solid sphere holding the stars, now used poetically for the visible heavens. Where sky is simply there, firmament carries cosmology, the sense of an ordered architecture above.

From Latin firmāmentum “a support,” from firmus “firm.” The word entered English through biblical and medieval cosmological language, where the heavens were imagined as structure, not emptiness: something that could “hold.”