Definition
A fluid whose volume does not change appreciably when pressure is applied to it. Liquids such as hydraulic fluid are treated as incompressible because squeezing them produces almost no reduction in volume, which is why they can transmit force efficiently through a closed system.
Plain English
A fluid that cannot be squeezed into a smaller space. When you push on it, it pushes back and moves -- it does not shrink.
Context Anchor
Seen in aircraft hydraulic system discussions, especially brakes, landing gear, and other parts moved by liquid pressure.
Derivation
From Latin 'in-' (not) and 'comprimere' (to press together). So 'incompressible' literally means 'cannot be pressed together' -- which is exactly what the term describes.
Why Pilots Care
This assumption simplifies lift and drag calculations for aircraft operating below roughly Mach 0.3.
Analogy
A completely full syringe with no air inside is a good picture: when you push one end, the liquid passes that push along instead of simply shrinking smaller.
Grounding Statement
In an aircraft hydraulic system, the fluid acts like a solid link for pressure even though it is a liquid.
Intuition Check
Do not read “incompressible” as meaning absolutely impossible to compress under any condition. In aviation use, it means the fluid compresses so little under normal system pressure that it can be treated as not compressing.
Example Sentence 1
Hydraulic brake systems rely on an incompressible fluid to transfer pedal pressure directly to the brake calipers.
Example Sentence 2
The student learned that once speeds rise, the incompressible fluid model no longer holds and compressibility effects must be considered.