Expanse or vault over the earth; sky.

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 theFIRMAMENT 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. ( Genesis 1:6-8)