Definition
The gaseous form of a substance that is normally a liquid or solid at ordinary temperatures and pressures. Vapor exists in the air alongside true gases, but unlike a permanent gas it can readily change back into its liquid or solid state with a change in temperature or pressure.
Plain English
Vapor is the gas form of something that is usually a liquid or solid. Water in the air is a common example — it can turn back into liquid water when it cools.
Context Anchor
Pilots encounter this word in weather discussions, fuel-system descriptions, engine starting, refueling, and fire-safety warnings.
Derivation
From the Latin vapor, meaning 'steam' or 'exhalation.' The original sense — visible steam rising from hot water — is a good mental anchor: a liquid quietly turning into gas.
Why Pilots Care
Water vapor drives nearly every weather phenomenon a pilot deals with — clouds, fog, icing, thunderstorms, and density altitude. Fuel vapor matters too: vapor lock can interrupt fuel flow on a hot day.
Grounding Statement
Picture steam rising off a cup of coffee — that visible wisp is water turning from liquid into vapor and mixing with the air.
Intuition Check
Vapor does not always mean something you can see. True vapor may be invisible; visible fog or mist is usually tiny liquid droplets suspended in the air.
Example Sentence 1
Warm air can hold more water vapor than cold air, which is why dew forms as temperatures drop overnight.
Example Sentence 2
The mechanic checked for vapor lock after the engine failed to restart on a hot day.