Definition
Substances that flow and conform to the shape of their container. In physics and aviation, the term covers both liquids and gases, since both yield to applied force and move freely under pressure differences.
Plain English
Anything that flows. This includes liquids like fuel, oil, and hydraulic fluid, as well as gases like air. If it can be pushed through a hose, line, or duct, it counts as a fluid.
Context Anchor
Seen in discussions of airflow, aircraft systems, engines, fuel, oil, and hydraulic systems.
Derivation
From the Latin fluidus, meaning 'flowing.' The aviation usage keeps that core idea: a fluid is anything that flows, which is why air is treated as a fluid in aerodynamics.
Why Pilots Care
Proper fluid levels and condition are vital to prevent engine failure, loss of control surfaces, or brake malfunction.
Grounding Statement
Air flowing over a wing and fuel flowing through a line are both fluids because both can move and flow.
Intuition Check
Do not read fluids as liquids only. In aviation and physics, gases such as air are also fluids because they flow.
Example Sentence 1
Hydraulic systems use a pressurized fluid to move flight controls, landing gear, and brakes.
Example Sentence 2
Low hydraulic fluid can lead to unresponsive flight controls.