Plain English Meaning
Skin, lips, or fingernails turning a bluish color because the blood does not have enough oxygen.
Full Definition
A bluish discoloration of the skin, fingernails, and lips caused by insufficient oxygen in the blood. In aviation medicine, it is recognized as a visible late-stage symptom of hypoxia.
Context Anchor
Seen in aeromedical discussions, especially when learning the symptoms of hypoxia at altitude.
Derivation
From the Greek 'kyanos' meaning 'dark blue,' plus the suffix '-osis' meaning 'condition.' Literally 'a blue condition' — which is exactly what a pilot would see on the fingernails or lips of someone running short on oxygen.
Why Pilots Care
It is an early visible sign of hypoxia that signals the need to descend or use supplemental oxygen before judgment and consciousness are lost.