A rebreather is a type of breathing apparatus that provides a breathing gas containing oxygen and recycles exhaled gas. This recycling reduces the volume of breathing gas used, making a rebreather lighter and more compact than an open-circuit breathing set for the same duration.
As a person breathes, the body consumes oxygen and makes carbon dioxide. A person with an open-circuit breathing set typically only uses about a quarter of the oxygen in the air that is breathed in. The rest is breathed out along with nitrogen and carbon dioxide.
With a rebreather, the exhaled gas is not discharged to waste. The rebreather recovers the exhaled gas for re-use. It absorbs the carbon dioxide, which otherwise would accumulate and cause carbon dioxide poisoning. It adds oxygen to replace what was consumed. Thus, the gas in the rebreather's circuit remains breathable and supports life processes. Nearly always, the oxygen comes from a gas cylinder, and the carbon dioxide is absorbed in a canister full of some absorbent chemical designed for diving applications such as Sofnolime, Dragersorb or Sodasorb. Some systems also use a prepackaged Reactive Plastic Curtain (RPC) based cartridge, a common brand name for these RPC cartridges is ExtendAir. These absorbents may contain small amounts of soda lime, but are generally less toxic. Pure oxygen is not considered to be safe for recreational diving below 6 meters, so recreational rebreathers also have a diluent cylinder to reduce the percentage of oxygen breathed and enable them to be used to greater depths.
Advantages of rebreather diving:
The main advantage of the rebreather over other breathing equipment is the rebreather's economical use of gas. With "open circuit" scuba, the entire breath is expelled into the surrounding water when the diver exhales. A breath inhaled from an open circuit scuba system whose cylinder(s) are filled with ordinary air is about 21% oxygen. When that breath is exhaled back into the surrounding environment, it has an oxygen level in the range of 15 to 16% when the diver is at atmospheric pressure.  This leaves the available oxygen utilization at about 25%; the remaining 75% is lost.
At depth, the advantage of a rebreather is even more marked. The amount of CO2 in exhaled gas is not a constant percentage, but a constant partial pressure of about 0.04bar. The amount of oxygen used from each breath is about the same - so as the ambient pressure increases (as a result of going deeper), the percentage of oxygen used from each breath drops. At 30m (100ft), a diver's exhaled breath contains about 20% oxygen and about 1% CO2.
Long or deep dives using open circuit equipment may not be feasible as there are limits to the number and weight of diving cylinders the diver can carry. The economy of gas consumption is also useful when the gas mix being breathed contains expensive gases, such as helium. In normal use only oxygen is consumed: small volumes of expensive inert gases can be reused for many dives.
Rebreathers produce far fewer bubbles and make less noise than open-circuit scuba; this can conceal military divers and allow divers engaged in marine biology and underwater photography to avoid alarming marine animals and thereby get closer to them. The electronic fully closed circuit rebreather, is able to minimise the proportion of inert gases in the breathing mix, and therefore minimise the decompression requirements of the diver, by maintaining a specific and relatively high oxygen partial pressure at all depths. The breathing gas in a rebreather is warmer and more moist than the dry and cold gas from open circuit equipment making it more comfortable to breathe on long dives and causing less dehydration in the diver.