Definition
Any substance that flows and conforms to the shape of its container. In aviation maintenance, the term covers both liquids (such as hydraulic oil, fuel, and engine oil) and gases (such as air, nitrogen, and oxygen), since both flow and transmit pressure.
Plain English
A fluid is anything that flows. That includes liquids you can pour and gases you can compress — both behave as fluids in aircraft systems.
Context Anchor
Seen in aircraft systems discussions, especially hydraulic, fuel, lubrication, pneumatic, and environmental systems.
Derivation
From the Latin fluidus, meaning 'flowing,' from fluere, 'to flow.' The aviation use keeps that core idea: if it flows, it's a fluid — whether liquid or gas.
Why Pilots Care
Leaks, contamination, or incorrect fluid types can cause hydraulic or fuel system failures that directly affect aircraft control and safety.
Intuition Check
Do not assume fluid means liquid only. In aviation maintenance, a fluid can be either a liquid or a gas if it flows through or acts within a system.
Example Sentence 1
Hydraulic systems use a fluid to transmit force from the cockpit controls to the landing gear and brakes.
Example Sentence 2
Air acts as a fluid when it moves through the pitot-static system.