Thayer's Greek Lexicon

 1. that which has been made firm
  1) the firmament, the arch of the sky, which in early times
was thought to be solid
1a1. a fortified place
  2) that which furnishes a foundation
1b1. on which a thing rests firmly, support
  3) firmness, steadfastness
1c1. metaph. in a military sense: solid front