Trapdoor

Hatches or trapdoors are a new block type that are going to be in the 1.6 update. They will function the same as doors, but are oriented such that they will be placed on a flat, horizontal surface. Like doors, they require a solid block next to them to be placed, and also do not allow water to pass through. Unlike doors, hatches only occupy a single block of space, and they open upwards.

Trivia
Much like the piston is based on a mod developed by Hippoplatimus, the hatch is very likely based on a mod from DJoslin. (http://www.djoslin.info/trapdoor.php)