All Liquids Are Fluids, but Not All Fluids Are Liquids
A fluid is a substance that flows. In physics the word covers gases as well as liquids, which is why the two are not synonyms.
A fluid is any substance that flows and takes the shape of its container. Everyday English treats fluid and liquid as the same, but physics does not: a fluid is anything that cannot resist a shearing force, which includes gases. Air is a fluid, and *fluid dynamics* covers aircraft as much as pipes. As an adjective, fluid also means changeable, as in a fluid situation.
- A flowing substance. *Brake fluid.*
- Changeable. *A fluid situation.*
- Smooth and graceful. *Fluid movement.*
- Fluid dynamics — the study of flow. *Fluid dynamics.*
- Liquid — one kind of fluid. *A liquid.*
Word forms
| Form | Word | Example |
|---|---|---|
| Noun | fluid | Clear fluid. |
| Plural | fluids | Body fluids. |
| Adjective | fluid | Fluid motion. |
| Noun | fluidity | Great fluidity. |
| Latin | fluere | To flow. |
- Check the brake fluid level.Mechanical.
- The plan remains fluid.Changeable.
- Air is a fluid in physics.Technical.
The fluere family
- Fluid — that which flows
- Influence — originally a flowing in, from the stars
- Affluent — flowing towards, hence wealthy
- Fluent — flowing speech
- Superfluous — overflowing, hence unnecessary
Latin *fluidus*, flowing, from *fluere*, to flow. English recorded it in the 1500s.
Is a gas a fluid?
In physics, yes. A fluid is anything that flows and cannot resist shear, which includes both gases and liquids.
Are influence and fluid related?
Yes. Influence originally meant a flowing in of power from the stars, from the same Latin verb.