Sump
From The Aquarium Wiki
The sump is a remote tank. It can serve many purposes, but its primary purpose is to provide extra water volume for the display. With extra water volume comes better stability. The sump can also be used to hide heaters, and filtration equipment under the stand. Typically a gravity/siphon system is used with an overflow system to get the water to the sump and a return pump is used to get the water circulated back to the display. When setting up a sump be sure that it is not so full that in the case of a power outage that it won't be able to absorb at least 3% of your display tank's water.
