'Firmament' definitions:

Definition of 'firmament'

noun

The apparent surface of the imaginary sphere on which celestial bodies appear to be projected [syn: celestial sphere, sphere, empyrean, firmament, heavens, vault of heaven, welkin]

From: GCIDE

Firmament \Fir"ma*ment\, n. [L. firmamentum, fr. firmare to make firm: cf. F. firmament. See Firm, v. & a.]
1. Fixed foundation; established basis. [Obs.] [1913 Webster]
Custom is the . . . firmament of the law. --Jer. Taylor. [1913 Webster]
2. The region of the air; the sky or heavens. [1913 Webster]
And God said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters. --Gen. i. 6. [1913 Webster]
And God said, Let there be lights in the firmament. --Gen. i. 14. [1913 Webster]
Note: In Scripture, the word denotes an expanse, a wide extent; the great arch or expanse over out heads, in which are placed the atmosphere and the clouds, and in which the stars appear to be placed, and are really seen. [1913 Webster]
3. (Old Astron.) The orb of the fixed stars; the most remote of the celestial spheres. [1913 Webster]

Firmament from the Vulgate firmamentum, which is used as the translation of the Hebrew _raki'a_. This word means simply "expansion." It denotes the space or expanse like an arch appearing immediately above us. They who rendered _raki'a_ by firmamentum regarded it as a solid body. The language of Scripture is not scientific but popular, and hence we read of the sun rising and setting, and also here the use of this particular word. It is plain that it was used to denote solidity as well as expansion. It formed a division between the waters above and the waters below (Gen. 1:7). The _raki'a_ supported the upper reservoir (Ps. 148:4). It was the support also of the heavenly bodies (Gen. 1:14), and is spoken of as having "windows" and "doors" (Gen. 7:11; Isa. 24:18; Mal. 3:10) through which the rain and snow might descend.