Genesis 1:6 KJV And God said, Let there be a firmament (H7549) in the midst of the waters, and let it divide the waters from the waters.
Other versions have "expanse." Firmament entered English translations because the Latin Bulgate has "firmamentum."
Genesis 1:6 YLT And God saith, 'Let an expanse be in the midst of the waters, and let it be separating between waters and waters.'
"From H7554; properly an expanse, that is, the firmament or (apparently) visible arch of the sky"