Definition
A type of aviation oxygen mask in which exhaled air is captured in a rebreather bag, allowing the oxygen content remaining in the user's exhaled breath to be mixed with fresh oxygen and reused on the next inhalation. Generally used at lower altitudes where 100 percent oxygen is not required.
Plain English
An oxygen mask with a small bag attached. When you breathe out, some of your breath collects in the bag and gets mixed with new oxygen, so you breathe it in again. This stretches the oxygen supply at lower altitudes.
Context Anchor
Seen in aircraft oxygen system descriptions, passenger oxygen equipment, and maintenance checks of masks, bags, hoses, and vents.
Derivation
From 're-' (again) and 'breathe' -- literally a mask that lets you breathe the same air again. The name describes exactly what the bag does: it holds exhaled air so it can be breathed a second time.
Why Pilots Care
Extends limited oxygen supply, reducing the need for larger tanks or more frequent servicing.
Intuition Check
Do not assume “rebreather” means the mask safely recycles every part of the breath. In this aircraft mask, it mainly reuses leftover oxygen while excess exhaled air must still be able to leave through vents.
Example Sentence 1
The light aircraft was equipped with rebreather oxygen masks for use during cruise above 12,500 feet.
Example Sentence 2
During preflight the technician verified the rebreather oxygen mask bag inflated and deflated smoothly with each breath.