Diamond,
According to Chris
[hr]
, here is the definition "A refugium is a body of water that's physically separated from the main aquarium, but shares a common water supply with it. (One) purpose of a refugium is to provide a refuge for animals and(/or) algae that suffer from excess predation in the main display tank. The (net) effect is to increase the diversity of the whole system beyond that possible with a single tank of similar size."
Again, a lot of people use it to help keep down nitrates. I hope this helps.