Air

Air is an invisible block used to define unoccupied space where other blocks and items could exist; it represents the absence of a block or an item. It is one of the few blocks in the game that players cannot normally interact with.

Cave air‌[Java Edition only] is a variant found only underground in carver caves[verify for Java Edition] and some generated structures along with features like lava lakes.

Void air‌[Java Edition only] is a variant used internally for the 'air' in unloaded chunks and outside of the world height limits (above Y=319 and below Y=-64 in the Overworld, and above Y=255 and below Y=0 in the Nether and the End dimensions).