For additional meanings of "Sky", please see Sky.
The sky was the part of the atmosphere or of outer space visible from the surface of a planet.
When Captain Jonathan Archer told the Novans Nadet and Jamin about the asteroid hit on Terra Nova in 2083, Jamin accused Archer to lie and the fact that "giant rocks" fall from the sky as shale. (ENT: "Terra Nova")
The pre-industrial society of Barkon IV believed that sky was one of the four basic elements of the universe, along with fire, water and rock. Sky was believed to be found in everything in existence, and could often be seen as smoke. (TNG: "Thine Own Self")
In 2369, sky was a word which appeared on a medical analysis, seen by Doctor Julian Bashir who was infected by the aphasia virus. (DS9: "Babel")