Definition
A flexible bag attached to a continuous-flow oxygen mask that collects oxygen from the supply line between the pilot's breaths, then delivers that stored oxygen during inhalation. This allows the user to inhale a larger volume of oxygen than the steady flow rate alone would provide.
Plain English
A small bag on the oxygen mask that fills with oxygen between breaths so you get a bigger gulp of it when you breathe in.
Context Anchor
Seen when studying or using a continuous-flow oxygen system, especially with passenger or pilot oxygen masks at higher altitudes.
Derivation
Reservoir comes from the French réservoir, meaning a place where something is stored or held in reserve. The bag holds oxygen in reserve between breaths so it's ready when you inhale.
Why Pilots Care
It increases the fraction of inspired oxygen at altitude, reducing the risk of hypoxia without requiring higher flow rates.
Grounding Statement
The reservoir bag catches oxygen during the part of the breathing cycle when you are not inhaling, then makes that oxygen available for the next breath.
Intuition Check
Do not think of the reservoir bag as the oxygen tank. It is only a small temporary holding bag at the mask; the actual oxygen supply is stored elsewhere in the aircraft.
Example Sentence 1
Before donning the mask, the pilot checked that the reservoir bag inflated normally as oxygen flowed into it.
Example Sentence 2
At 12,000 feet the pilot relied on the reservoir bag to maintain adequate oxygen concentration without increasing the flow setting.