actually...a lot of sumps don't contain filter media at all anymore...the majority of people use live rock for their biological filtration and use the sump as a place to add either more LR, a DSB, or simply as a place for equipment. my sump, for instance, is simply for my heater, skimmer, and thermometer...and to add an additional 20g of volume to the system
sump just means a big hole to put liquid in...its an old term adopted for aquarium use...mines use them for liquid runoff to prevent cave-ins...your cars oil pan is also a sump, and it actually works very much along the same principles but for different purposes