A riggers belt is a very strong wide pants belt that holds your pants up. Generally it is about 1.75" wide, made up of two layers of very strong webbing, and has a steel D sewn into the side of the buckle to attach a carabiner. Not for climbing, but the origin was in the US Army I think, occasionally soldiers would have to cross a fast running stream via a rope stretched out across it, and sometimes guys would fall in, point being a couple had drowned. So the parachute riggers were approached to make belts for the guys so they could attach a carabiner to the belt to help hold them to the rope. Is a safety item, and as a last resort rappelling device (best used in conjuction with a rope "Swiss Seat" harness).
The one I have:
http://tagmulticam.com/heavydutyriggersbelt.aspx