1. The vault of heaven, the sky.
2. The upper air.
Etymology: probably from Old English wolcen, cloud.
Used by Shakespeare ("your welkin eye", "shall we make the welkin dance", etc.)
and Milton ("from either end of heaven the welkin burns").
(The phrase "welkin eyes" refers to blue eyes, like the sky.)
"Make the welkin ring" means to make so much noise that it echoes in the clouds.
Charles Stubbs used the phrase "let the welkin ring" in 'The Carol of King Cnut'.