King James Bible Dictionary

From the Vulgate firmamentum, which is used as the translation of the Hebrew raki'a. This word means simply "expansion." It denotes the space or expanse like an arch appearing immediately above us. They who rendered raki'a by firmamentum regarded it as a solid body. The language of Scripture is not scientific but popular, and hence we read of the sun rising and setting, and also here the use of this particular word. It is plain that it was used to denote solidity as well as expansion. It formed a division between the waters above and the waters below (Genesis 1:7). The raki'a supported the upper reservoir (Psalms 148:4). It was the support also of the heavenly bodies (Genesis 1:14), and is spoken of as having "windows" and "doors" (Genesis 7:11; Isaiah 24:18; Malachi 3:10) through which the rain and snow might descend.

The expanse above the earth.
Genesis 1:6-8; Genesis 1:14-17; Genesis 1:20; Psalms 19:1; Daniel 12:3

In Scripture the word denotes an expanse, a wide extent; for such is the signification of the Hebrew word. The original, therefore, does not convey the sense of solidity, but of stretching, extension; the great arch of expanse over our heads, in which are placed the atmosphere and the clouds, and in which the stars appear to be placed, and are really seen.

Webster.

FIRMAMENT, noun ferm'ament. [Latin firmamentum, from firmus, firmo.]

The region of the air; the sky or heavens. In scripture, the word denotes an expanse, a wide extent; for such is the signification of the Hebrew word, coinciding with regio, region, and reach. The original therefore does not convey the sense of solidity, but of stretching, extension; the great arch or expanse over our heads, in which are placed the atmosphere and the clouds, and in which the stars appear to be placed, and are really seen.

And God said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters. Genesis 1:6.

And God said, Let there be lights in the firmament Genesis 1:14.

FIRMAMENT'AL, adjective Pertaining to the firmament; celestial; being of the upper regions.