Plain English Meaning

Air moves and flows around things much like water does. Even though we can't see it, it pushes, slides, and curves around objects, which is why an airplane can be lifted and steered by it.

Full Definition

A foundational principle in aerodynamics stating that air, although invisible and made of gas, behaves as a fluid because it flows, deforms around objects, and exerts pressure in the same way liquids do. This means air can be described and analyzed using the same physical laws that govern the motion of water and other fluids, including pressure, density, viscosity, and flow patterns around a body moving through it.

Context Anchor

Seen in basic aerodynamics when learning how wings produce lift and how the airplane moves through the air.

Derivation

The word 'fluid' comes from the Latin 'fluidus,' meaning 'flowing.' In , a fluid is anything that flows and takes the shape of its container -- which includes liquids and gases. Knowing this helps why air, despite feeling like 'nothing,' behaves predictably enough to lift an .