Plain English Meaning

It's the way your body slowly gets used to thinner air when you spend time at higher altitudes. A pilot who lives at sea level and suddenly flies high will feel the effects of low oxygen much sooner than someone whose body has had time to adapt.

Full Definition

The gradual physiological adjustment a pilot's body makes to reduced oxygen levels at higher altitudes over a period of days or weeks, allowing more efficient oxygen use at altitude. In aviation, an unacclimatized pilot is more susceptible to hypoxia, fatigue, and impaired judgment at altitudes where an acclimatized person might still function adequately.