Boyd’s Bible Dictionary:
(made firm). Overhead expanse (Gen. 1:17); solid (Ex. 24:10); with windows and doors (Gen. 7:11; Isa. 24:18; Psa. 78:23).
Concise Bible Dictionary:
The Hebrew word is raqia, signifying “expanse.” It is used for the celestial sphere that may be seen by looking upward, and also simply for the atmosphere in which the birds fly. We read that God called the firmament “heaven:” this is “heaven” in a broad sense as we read elsewhere of “the stars of heaven,” but also of “the birds of heaven” (Gen. 1:6-20). The Psalmist speaks of them as distinct: "The heavens declare the glory of God; and the firmament showeth his handywork" (Psa. 19:1; Psa. 150:1). The living creatures in Ezekiel 1 move amidst the firmament: "and the likeness of the firmament upon the heads of the living creature was as the color of the terrible crystal, stretched forth over their heads above" (Ezek. 1:22), showing them to be executors of God's judicial government: (Compare Ezek. 10:1).
Strong’s Dictionary of Hebrew Words:
Meaning:
from 7554; properly, an expanse, i.e. the firmament or (apparently) visible arch of the sky