Plain English Meaning

A fluid is anything that flows. That includes liquids you can pour and gases you can compress — both behave as fluids in aircraft systems.

Full 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.

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 idea: if it flows, it's a fluid — whether liquid or .