Definition
To draw off a portion of a fluid, gas, or pressure from a system. In aviation, the term commonly refers to extracting compressed air from a turbine engine's compressor section for use in other aircraft systems, or to releasing trapped air or fluid from a hydraulic or fuel line so the system can operate correctly.
Plain English
To take some air, gas, or liquid out of a system on purpose -- either to use it somewhere else, or to remove unwanted air that's gotten trapped inside.
Context Anchor
Seen in turbine-engine systems, cabin pressurization and heating, anti-ice systems, and brake or hydraulic maintenance.
Derivation
From Old English 'bledan,' meaning to let blood. The medical sense of letting fluid out of a body was extended to mechanical systems -- letting fluid or pressure out of a closed system in a controlled way.
Why Pilots Care
Bleed air powers important systems like cabin pressurisation, anti-ice, and engine starting. Knowing what bleed air is doing -- and when to turn it off -- matters for engine performance and troubleshooting. On the maintenance side, air left trapped in hydraulic lines (like the brakes) makes them feel spongy or fail outright, so bleeding the system is a routine task.
Intuition Check
Bleed does not mean a leak by accident here. In aviation, bleed usually means a controlled, intentional draw-off of air or fluid.
Example Sentence 1
The mechanic bled the brake lines after replacing the hydraulic fluid.
Example Sentence 2
Bleed air from the engines keeps the cabin pressurized at altitude.