Definition
A flexible reservoir bag attached to a continuous-flow oxygen mask that captures the user's exhaled breath from the upper airway and combines it with incoming oxygen, allowing the unused oxygen from the previous breath to be inhaled again on the next breath.
Plain English
A small bag on the oxygen mask that catches the air you breathe out and lets you breathe some of it back in, so the oxygen that wasn't used the first time isn't wasted.
Context Anchor
Seen on continuous-flow oxygen masks used in unpressurized aircraft at higher altitudes.
Derivation
From 're-' (again) and 'breathe' -- literally a bag for breathing again. The name describes the function: exhaled air is held in the bag and re-inhaled rather than being lost.
Why Pilots Care
It conserves the aircraft’s limited oxygen supply, extending the time the system can be used before the cylinder is depleted.
Analogy
It is like a small holding pouch for oxygen. The oxygen keeps arriving even while you are not inhaling, so the bag saves it for your next breath.
Intuition Check
Do not assume “rebreather” means you are simply breathing old, used-up air. In this system, the bag mainly stores fresh oxygen between breaths and may reuse only a small amount of still-usable air from the last breath.
Example Sentence 1
Before climbing above 12,500 feet, the pilot showed each passenger how the rebreather bag should partially inflate and deflate with every breath.
Example Sentence 2
With each breath the rebreather bag inflated and partially deflated, allowing the pilot to reuse some of the delivered oxygen.