What is the definition or description of: high altitude?

High altitude: Simply put, literally, 'high altitude'. As one goes higher than the 'sea level', be it on mountains or aircraft of some sort, the air becomes 'rarer' or 'less denser'. Higher one goes, more likely the loss in density in air, and higher the chances of having breathing complications.

Spaced out Air!: As one ascends through the atmosphere, every breath contains fewer molecules of oxygen. One must work harder to obtain oxygen, by breathing faster and deeper. The % of oxygen in the atmosphere at sea level is 21%. As altitude increases, the % remains the same but the number of oxygen molecules per breath is reduced. At 12, 000 feet 40% fewer oxygen molecules per breath. Your body has to cope!