Definition
The gaseous form of a substance that is normally a liquid or solid at ordinary temperatures and pressures. A vapour can be returned to its liquid or solid state by cooling it or by increasing the pressure on it.
Plain English
The gas form of something that is usually a liquid or solid. Water turning into steam is a vapour. Cool it or squeeze it hard enough and it goes back to liquid.
Context Anchor
Pilots most often meet this word in weather discussions involving water vapour and in fuel-system discussions involving fuel vapour.
Derivation
From the Latin vapor, meaning steam or warm exhalation. The original sense was visible mist rising off something warm — which captures the everyday image of vapour well.
Why Pilots Care
Vapour formation in fuel lines can interrupt flow to the engine and cause power loss.
Grounding Statement
If liquid fuel gives off gas inside a tank, that gas is fuel vapour.
Intuition Check
Vapour does not always mean something you can see. Water vapour itself is invisible; clouds, fog, and mist are tiny water droplets or ice particles suspended in the air.
Example Sentence 1
On a hot day with low fuel pressure, fuel vapour can form in the lines and cause the engine to run rough.
Example Sentence 2
On a hot day the pilot watched for signs of vapour lock that could stop fuel from reaching the engine.