Plain English Meaning
When air sinks, the higher pressure below squeezes it, and squeezing air makes it warmer. The warming comes from the compression itself, not from any outside heat source.
Full Definition
The increase in temperature of a parcel of air caused by compression as it descends to lower altitudes, with no heat added from or lost to the surrounding air.
Context Anchor
Seen in weather and atmospheric stability discussions, especially when studying rising and sinking air.
Derivation
From the Greek 'adiabatos', meaning 'not to be passed through' — referring to the idea that no heat passes in or out. The temperature change comes entirely from compression, not from heat flowing in from outside.
Why Pilots Care
Affects forecasts of temperature, cloud formation, and in descending air, which can influence and encounters.