The word "firmament" that appears in the Bible is a reference to the expanse of heavens that are above the Earth. The word appears nine times in the Book of Genesis.
Firmament worked to form a division between the waters below and water above. In Genesis 1:6-8, the word is referenced in the sentence, "Let there be a firmament in the midst of the water, and let it divide the water from the waters." The word is used 17 times in the Bible, including in Psalms. In Ezekiel, firmament appears five times and only once in Daniel. Firmament is only used in older translations of the Bible.