Self-contained
breathing device for
underwater use that reuses at least part of each
breath.
Scuba, in contrast, expells the entire
breath into the surrounding
water.
Rebreathers are able to reuse the oxygen not used by your body during each breath and remove CO2 with a chemical that acts as a "trap" agent. This results in extended dives with a small tank. Also, a rebreather produces no bubbles and thus a much quieter dive.
There are basically two classes of rebreathers: The semi-closed and the fully-closed. Semi-closed rebreathers expel a part of the breathing gases, whereas fully-closed rebreathers are completely self-contained. Both models, however, need to expel gases during the ascent, to maintain proper gas pressure in the system.
Some rebreathers allow a diver to stay underwater with very little gas for over eight hours.