An abstract sphere of infinite radius which serves as the imaginary backdrop for celestial objects, and of which the visible sky is one hemisphere.
the figure of the universe as an imaginary sphere whose center is the Earth
In the geocentric Ptolemaic system, the eighth celestial sphere which carried the fixed stars; (countable, by extension) any celestial sphere.
The abode of God and the angels; heaven.
The vault of the heavens, where the clouds, sun, moon, and stars can be seen; the heavens, the sky.
The field or sphere of an activity or interest.