Definition
The portion of total atmospheric pressure that is contributed by oxygen alone. Air is a mixture of gases, and each gas exerts its own pressure independently; the partial pressure of oxygen is the share of the total pressure produced by the oxygen molecules in that mixture. As altitude increases, total atmospheric pressure decreases, and so does the partial pressure of oxygen — even though the percentage of oxygen in the air (about 21%) remains the same.
Plain English
It's the amount of pressure that the oxygen part of the air is pushing with. The higher you go, the less it pushes — which is why your body struggles to absorb oxygen at altitude even though the air around you still contains the same percentage of oxygen.
Context Anchor
Seen in high-altitude and pressurized-aircraft discussions, especially when explaining why pilots can become short of oxygen as air pressure decreases.
Derivation
From Latin pars (part) and pressura (pressing). 'Partial' means belonging to one part of the whole — here, the part of total air pressure attributable just to oxygen. This origin reinforces the key idea: it isn't oxygen's amount, it's oxygen's share of the pressure.
Why Pilots Care
As altitude increases, total air pressure falls and the partial pressure of oxygen drops, reducing the amount of oxygen the body can absorb and raising hypoxia risk.
Grounding Statement
Even though the air at 18,000 feet still contains 21% oxygen, the oxygen is pushing only half as hard as it does at sea level — and that 'push' is what gets it into your blood.
Intuition Check
Do not confuse partial pressure of oxygen with oxygen percentage. The air can still be about 21 percent oxygen, while the partial pressure of oxygen is too low for safe pilot performance.
Example Sentence 1
As the aircraft climbed above 12,500 feet, the reduced partial pressure of oxygen began to affect the pilot's reaction time and judgment.
Example Sentence 2
The aircraft pressurization system keeps cabin altitude low enough to maintain a safe partial pressure of oxygen for all occupants.