Definition
A colorless, odorless, tasteless gas that makes up about 21 percent of the Earth's atmosphere by volume and is essential for human respiration and for combustion in piston and turbine engines.
Plain English
The gas in the air that people need to breathe and that engines need to burn fuel.
Context Anchor
Seen in atmosphere, altitude, night flying, and hypoxia discussions, especially when deciding whether supplemental oxygen is needed.
Derivation
From the Greek 'oxys' (sharp, acid) and 'genes' (forming) — literally 'acid-former.' Early chemists thought oxygen was the key ingredient in all acids. The name stuck even after that idea was disproved. Useful for pilots because it's a reminder that oxygen is a chemically active gas, not just inert air.
Why Pilots Care
Reduced oxygen availability at altitude causes hypoxia that impairs judgment and can lead to loss of consciousness.
Grounding Statement
At sea level there is plenty of oxygen to breathe; the higher you climb, the less of it reaches your lungs with each breath, even though the percentage in the air stays the same.
Intuition Check
Oxygen does not mean the same thing as air. Air is a mixture of gases; oxygen is the part your body needs most for clear thinking and normal function.
Example Sentence 1
Above 12,500 feet, the pilot switched on supplemental oxygen to avoid the effects of hypoxia.
Example Sentence 2
Before an instrument flight above 10,000 feet, the crew verified the aircraft oxygen system was fully charged.