Definition
A fluid whose volume decreases noticeably when pressure is applied to it. All gases are compressible fluids; their density changes significantly with changes in pressure and temperature.
Plain English
A fluid you can squeeze into a smaller space by pushing on it. Air and other gases behave this way — push hard enough and the same amount of gas takes up less room.
Context Anchor
Seen in aerodynamics, aircraft performance, engine, and high-speed airflow discussions.
Derivation
From Latin 'comprimere,' meaning 'to press together.' A compressible fluid is one that can literally be pressed into a smaller volume.
Why Pilots Care
At higher speeds, air no longer behaves as an incompressible fluid; density changes create shock waves, drag rise, and control changes that affect aircraft handling and performance limits.
Analogy
Think of a bicycle pump with your finger over the outlet. As you push the handle down, the air inside squeezes into a smaller volume — that's compressibility. Try the same thing with a syringe full of water and the plunger barely moves.
Grounding Statement
If you squeeze air into a smaller space, the same air takes up less room and its pressure rises.
Intuition Check
Do not read “fluid” as only meaning liquid. In aviation and physics, air is also a fluid because it flows.
Example Sentence 1
Because air is a compressible fluid, its density drops as the aircraft climbs into thinner atmosphere.
Example Sentence 2
The wing was designed with the compressible nature of air in mind to delay the onset of shock waves.