Word

fluid

noun C1

UK /ˈfluːɪd/

US /ˈflʉwɪd/

Meanings

noun

A substance (liquid or gas) that can flow and does not have a fixed shape.

noun

A liquid used in a machine or system, especially for cooling, lubrication, or operation.

adjective

Smooth, flowing, and easy; not stiff or awkward.

Definition

Fluid describes a substance that flows easily, such as a liquid or a gas.

A fluid is any substance that can flow and take the shape of its container, including liquids and gases. In everyday use, it often refers to liquids in machines or the body, such as brake fluid or cooling fluid. As an adjective, it can also describe movement, speech, or actions that are smooth and continuous.

Examples

In physics, we modeled air as a fluid to predict how it would move around the wing.
The technician checked the fluid level before restarting the machine to avoid overheating.
Her fluid presentation made complex ideas feel coherent rather than intimidating.
They adjusted the nozzle until the fluid streamed steadily without splashing.
I prefer a fluid workflow that lets me revise the plan as new information appears.

Common mistake

Learners often use “fluid” only to mean “liquid,” but it can also refer to gases and to smooth, flowing movement or speech.