Air

Air is a block in Minecraft used to represent unoccupied spaces.

Obtaining
Air cannot be kept in the inventory and can only be destroyed or created by the placement or removal of other blocks respectively.

Natural generation
Most generated structures generate air. Air is also generated in any space not occupied by another block.

Usage
Air, like any non-solid blocks, are spaces the player and other mobs are allowed to move within without suffocating.