Plain English Meaning
Not enough oxygen reaching your body and brain. As you fly higher, the air gets thinner and your body struggles to take in the oxygen it needs, which affects how clearly you can think, see, and react.
Full Definition
A condition in which the body's tissues, particularly the brain, do not receive enough oxygen to function normally. In aviation, it is most commonly caused by reduced atmospheric pressure at altitude, which lowers the amount of oxygen the lungs can absorb into the bloodstream.
Context Anchor
Seen in night flying and human factors discussions, especially when the handbook explains why altitude, fatigue, and oxygen supply affect vision.
Derivation
From Greek 'hypo-' meaning 'under' or 'below,' and '-oxia' from 'oxygen.' Literally 'below-oxygen.' The 'hypo-' prefix is a useful anchor — it shows up in other medical terms like 'hypothermia' (below ) and signals 'less than enough.'