The “firmament” (from the Latin firmamentum, meaning “sky” or “expanse”) is mentioned 17 times in the King James Version of the Bible and refers to the expanse of the heavens above the earth.
Nine of the occurrences of firmament are in the first chapter of the Bible as part of the creation account. Genesis 1:6-8 says, “And God said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters. And 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. And the evening and the morning were the second day.” The “firmament” is called “heaven”; i.e., it is what people see when they stand outside and look up. It is the space which includes the earth’s atmosphere and the celestial realm. In the firmament, we see the sun, moon, and stars; in modern translations the firmament is often called the “expanse” or the “sky.”
Genesis says that the firmament “separated the water under the expanse from the water above it” (Genesis 1:7). Originally, God created the earth with water “under” the sky (terrestrial and subterranean water) and water “above” the sky—possibly a “water canopy” which enwrapped the earth in a protective layer. Or, the waters above the firmament could simply be a reference to clouds.
We find firmament used again in Psalms: “The heavens declare the glory of God; and the firmament sheweth his handywork” (Psalm 19:1). Also, in Psalm 150:1, “Praise ye the LORD. . . . Praise him in the firmament of his power.”
Firmament is used in only two other books of the Bible: Ezekiel (five times) and Daniel (once). In Ezekiel, each occurrence takes place within a vision. For example, “Then I looked, and, behold, in the firmament that was above the head of the cherubims there appeared over them as it were a sapphire stone, as the appearance of the likeness of a throne” (Ezekiel 10:1).
Daniel 12:3 says, “And they that be wise shall shine as the brightness of the firmament; and they that turn many to righteousness as the stars for ever and ever.”
In short, the “firmament” is a vast expanse, specifically the atmosphere or sky. The word is found only in the King James Version and other older translations of the Bible.