Sky

The sky was the part of the atmosphere or of outer space visible from the surface of a planet.

The preindustrial civilization of Barkon IV incorrectly believed that sky was one of the four basic elements of the universe, along with fire, rock and water. Sky was believed to be found in everything in existence, and could often be seen as smoke. 

In 2369, sky was a word which appeared on a medical analysis, seen by Doctor Julian Bashir who was infected by the aphasia virus.