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

Genesis 1:7
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.

Genesis 1:8
And God called the firmament Heaven. And the evening and the morning were the second day.

Genesis 1:14
And God said, Let there be lights in the firmament of the heaven to divide the day from the night; and let them be for signs, and for seasons, and for days, and years:

Ezekiel 1:23
And under the firmament were their wings straight, the one toward the other: every one had two, which covered on this side, and every one had two, which covered on that side, their bodies.

Ezekiel 1:25
And there was a voice from the firmament that was over their heads, when they stood, and had let down their wings.

https://bible.knowing-jesus.com/words/Firmament