The dome-like expanse of sky that God created on day two and stretched out like a tent (Ps 104:2). The heavens declare God's glory (Ps 19:1) — the raqia itself is a proclamation.
Ancient cosmology pictured the raqia as a solid dome holding back upper waters. Theologically it represents God's ordering of chaos into creation — the sky as the boundary of habitable space.
Daniel 12:3 uses raqia to describe resurrection glory — the wise shining like the sky. NT resurrection glory (Matt 13:43: 'the righteous will shine like the sun') draws on this image.