Minecraft wiki entry for Development_versions, associated text: Redefined the concept of air.[8]
Is now an actual block instead of the absence of a block (null).[9]
It can be kept in the inventory.
It appears as a magenta and black checker pattern (the missing texture) in the inventory, when dropped, and when put into an item frame. But when held in the player's hand, it looks invisible, like an actual air block.
Drops from entities that would normally drop nothing, including villagers, bats, silverfish, and the ender dragon.