Answer:
In the Israelite account of creation found in the Book of Genesis, the expanse that separated water from water is referred to as the "firmament" or "vault of the sky." This concept represents the sky or atmosphere that was thought to separate the waters above (possibly clouds or celestial bodies) from the waters below (such as seas or oceans). It's a poetic and symbolic description of the divine act of creation. The imagery used in ancient texts like these can provide insights into the beliefs and cultural understanding of the time