Source : Webster's Revised Unabridged Dictionary (1913)

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.]

            Custom is the . . . firmament of the law. --Jer.
                                                  Taylor.

   2. The region of the air; the sky or heavens.

            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.

            And God said, Let there be lights in the firmament.
                                                  --Gen. i. 14.

   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.

   3. (Old Astron.) The orb of the fixed stars; the most rmote
      of the celestial spheres.

Source : WordNet®

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