Plain English Meaning
It is the type of oxygen starvation pilots experience when flying high. The air around you has plenty of oxygen, but the pressure is too low to push enough of it into your blood, so your body slowly starts running short.
Full Definition
A form of hypoxia caused by insufficient oxygen pressure in the lungs, which prevents enough oxygen from passing into the bloodstream. It is most commonly the result of reduced atmospheric pressure at altitude, where the air still contains the same percentage of oxygen but the lower pressure means fewer oxygen molecules are available with each breath.
Context Anchor
Seen in aeromedical and high-altitude flying discussions, especially when learning about oxygen use, pressurization, and pilot performance at altitude.