firmamentum.

From firmō (“strengthen”) + -mentum, from firmus (“firm”). Literally, “that which strengthens or supports”. The meaning of "a strengthening, support, prop", especially in the figurative sense (of an argument etc., τὸ συνέχον (tò sunékhon)) is classical, and frequently occurs in Cicero. Further details The cosmological sense is coined in the Vulgate in imitation of LXX στερέωμα (steréoma, “firm or solid structure”), which in turn translates Biblical Hebrew רקיע. Some have viewed this as a mistranslation and the modern New American Standard Bible translates this as "expanse" — based on a modern scientific bias. However, ancient Hebrews — including those who translated the Hebrew Bible into Greek (the Septuagint or LXX) — understood this term to refer to solid material. Hence, the LXX στερέωμα (steréōma, “firm or solid structure”). The Hebrew רקיע (raqia), refers to a hard substance that has been hammered out by crafstmen (Exodus 39:3; Isaiah 40:19; compare Job 37:18, Exodus 24:10, Ezek. 1:22). Ancient Hebrews, in line with their Mesopotamian neighbors, understood the "firmament" or "sky" as a dome, over which was a layer of water, under which rotated the sun, moon, and stars (Gen. 1:6-8, 14-15). The Genesis creation account is accommodated to this pre-scientific understanding of the sky as a dome, with a layer of cosmic waters above and the stellar constellations below.