Air

Air represents unoccupied spaces. Although it is internally treated as every other block, air cannot be kept in the inventory and can only be destroyed or created by the placement or removal of other blocks. Air and non-solid blocks are the only spaces the player and other mobs are allowed to move within without mods or glitches. The sole exception is the Ender Dragon which can move within Obsidian, Bedrock, and End Stone.

Appearance
Air is a completely invisible block.

Trivia

 * It is possible to survive without air, because other non-solid blocks (that aren't fluid blocks) won't drown or suffocate the player.
 * Air is the only gas in Minecraft.