The definition of the Firmament can be essentially summarized as the arch or vault over the earth and sky that separates the earthly realm from what is beyond.
Arch- a curved symmetrical structure spanning an opening and typically supporting the weight of a bridge, roof, or wall above it.
the firmament as “the vault or arch of the sky; heavens.”
Then God said, "Let there be a firmament in the midst of the waters, and let it divide the waters from the waters." Thus God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament; and it was so. And God called the firmament Heaven..." (Genesis 1:6-8 NKJV).
