Closed Loop?
 

What exactly is meant by the term "closed loop" ?

Meaning water goes directly from tank through pump and back to tank again. Closed loops do not run through a sump, and usually not through an overflow either. They are, like in the instance of my 150g tank, a bulkhead in the bottom of the tank plumbed directly, with shutoff and unions, to a pump's intake, then the pump outlet is fed directly back into the tank

^ Ya, basically an external powerhead. :D

personally its a cleaner way to move water rather then using power heads. JMHO

lots of pictures on OM's site (here) with pictures of tanks with CLs