Plain English Meaning
When a pilot's body is not getting enough oxygen even though they are flying at a fairly low altitude. It happens because something about the pilot's own condition—being tired, sick, dehydrated, a smoker, or affected by carbon monoxide—is making it harder for the body to use oxygen.
Full Definition
A reduced-oxygen condition affecting a pilot at altitudes well below those normally associated with hypoxia, caused by physiological factors such as fatigue, dehydration, illness, smoking, alcohol use, or carbon monoxide exposure rather than by reduced atmospheric pressure alone.