Plain English Meaning
Extra oxygen breathed in from a tank or system in the aircraft, used at high altitudes because the air up there is too thin for your body to get enough oxygen on its own.
Full Definition
Additional oxygen delivered to a pilot or passengers through a mask or cannula at higher altitudes, where the reduced atmospheric pressure no longer allows the body to absorb enough oxygen from the surrounding air to maintain normal function.
Context Anchor
Seen in night flying, high-altitude operations, and discussions of factors that affect vision and pilot performance.