Air

Air is an invisible non-solid block. However, it cannot be created, destroyed, or obtained, even with an inventory editor. Although technically a block, it is usually not seen as such. Air is unique in the way that it automatically replaces any block that is removed even though it doesn't visually flow. Air is what is required to not take suffocation damage, such as from Water or falling Sand/Gravel

Background
In the Indev versions, air affected fluid blocks that were horizontally adjacent to it. If air was touching the fluid block, there was a 2/3 chance the fluid block would disappear and make the pool recede and a 1/3 chance the fluid block fill the air, making the pool spread.

Trivia

 * Air is the #1 most common block in Minecraft, as it spreads everywhere where there are no other blocks.
 * Air is programmed to fill any adjacent blocks that are not full. Sometimes, there is a glitch where you take suffocation damage when blocking water, or emptying the block of anything other than air that you are standing in.