post #2857846 post #2837041

A rebreather is a breathing device that often gets interchanged with an oxygen mask or gas mask.
What differentiates a rebreather from a convential oxygen mask where pure oxygen is inhaled and consumed oxygen is expelled out of the breathing circuit, the user breathes through a mask or mouthpiece from a supply of oxygen, but the consumed oxygen is expelled back into the breathing circuit on exhalation and scrubbed from CO2 so it becomes relatively breathable again, as a means to conserve used oxygen.
Rebreathers are commonly used by divers at great depths to conserve as much oxygen as possible and minimize the chance of asphyxiation. a still present danger of rebreathers though is that consumed oxygen in a rebreather degrades as it gets recycled.

Note:

A rebreather is not the same thing as a rebreather bag.

Normally, characters that are underwater, or shown somewhere where they could easily dive underwater (on a boat, on the shore, etc), are using a rebreather. Real-life rebreathers look like a complicated backpack, with two relatively large hoses (40 mm / 1.5" or more diameter) that go from the backpack to a mouthpiece.

Characters that are in bondage - especially full rubber bondage - and not underwater, are probably using a rebreather bag as a form of breath play. Rebreather bags are usually depicted as a rubber bag, often black, about the size of a football (either kind). The bag has a single hose, usually about 25 mm / 1" diameter, usually connected to a mask over the character's mouth. In an animation, the bag will inflate and deflate as the character breathes.

Rebreather bag examples: post #3815523 post #2562954