empty space; void; empty sky
Refers to a vast, empty expanse, either the physical sky or a metaphorical void. Often carries a literary or philosophical tone.
He stared into empty space, lost in thought.
The bird disappeared into the empty sky.
Before he collapsed, he clutched at thin air.
The voice faded away into the void.
The old man stared into the void, saying nothing.
空 is the plain, everyday word for the physical sky, whereas 虚空 is more literary and can also mean an abstract, metaphysical void.
虚無 emphasizes philosophical nothingness or nihility, while 虚空 more often pictures an actual vast, empty space (like the sky).
Compound of 虚 (kyo, 'empty, void') and 空 (kū, 'sky, emptiness'). The reading こくう is the on'yomi of both characters.