Definition
The temperature at which a liquid changes into a vapor, where the vapor pressure of the liquid equals the surrounding atmospheric pressure. Because boiling point depends on surrounding pressure, the same liquid boils at a lower temperature when atmospheric pressure is lower (such as at altitude) and at a higher temperature when pressure is higher (such as in a pressurized cooling system).
Plain English
The temperature at which a liquid turns into a gas. It is not a fixed number for each liquid -- it changes with the air pressure around it. Lower pressure means the liquid boils at a lower temperature; higher pressure means it boils at a higher temperature.
Context Anchor
Seen in aircraft systems, fuel and fluid discussions, weather and altitude effects, and any explanation of how pressure changes affect liquids.
Derivation
From the Old French 'boillir,' meaning 'to bubble or churn.' The 'point' is simply the specific temperature at which that bubbling begins. The term reminds us we are looking for the exact moment a liquid starts converting to vapor.
Why Pilots Care
Lower atmospheric pressure at altitude reduces the boiling point of fuel, which can lead to vapor lock and engine power loss.
Analogy
Water boils at a lower temperature on a high mountain than it does at sea level because the surrounding air pressure is lower. The liquid did not change; the pressure around it did.
Grounding Statement
Water boils at 212°F at sea level, but on top of a high mountain it boils at a noticeably lower temperature -- because there is less air pressing down on it.
Intuition Check
Do not assume boiling point is always one fixed temperature. For aviation use, the key idea is that boiling point depends on the pressure around the liquid.
Example Sentence 1
At high altitudes, the lower atmospheric pressure reduces the boiling point of fuel, increasing the risk of vapor lock in the fuel lines.
Example Sentence 2
Hot weather combined with a low boiling point fuel can cause problems during ground operations on warm days.