空 - empty, sky, void

air defense; air defence

vacant land; unoccupied ground; empty lot

blue sky

vacant house; unoccupied house

wide open sky; the blue; heavens; firmament

(outer) space

airspace

airline ticket; plane ticket; air ticket

airmail; air mail

space; room; gap; emptiness

empty gun; gun loaded with blanks; firing a blank

fictitious; imaginary; fanciful; fabricated