fluid

American  

[floo-id] / ˈflu ɪd /

noun

1. a substance, as a liquid or gas, that is capable of flowing and that changes its shape at a steady rate when acted upon by a force tending to change its shape.

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adjective

1. pertaining to a substance that easily changes its shape; capable of flowing.

2. consisting of or pertaining to fluids.

3. changing readily; shifting; not fixed, stable, or rigid.

   fluid movements.

4. convertible into cash.

   fluid assets.

fluid British  

/ ˈfluːɪd /

noun

1. a substance, such as a liquid or gas, that can flow, has no fixed shape, and offers little resistance to an external stress

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adjective

1. capable of flowing and easily changing shape

2. of, concerned with, or using a fluid or fluids

3. constantly changing or apt to change

4. smooth in shape or movement; flowing

fluid Scientific  

/ flo̅o̅′ĭd /

1. A state of matter, such as liquid or gas, in which the component particles (generally molecules) can move past one another. Fluids flow easily and conform to the shape of their containers.

2. See also state of matter viscosity

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fluid Cultural  

1. In physics, a substance that flows — usually a liquid or a gas.

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Related Words

See liquid.

Other Word Forms

• fluidal adjective
• fluidally adverb
• fluidly adverb
• fluidness noun
• nonfluid noun
• nonfluidly adverb
• unfluid adjective

Etymology

Origin of fluid

First recorded in 1595–1605; from Latin fluidus, equivalent to flu(ere) “to flow” + -idus adjective suffix; see -id ⁴

Explanation

Something that is unstable or subject to change can be described as fluid. The fluid political situation in a particular country makes it unsafe to travel there, with the constant possibility of a violent uprising. A liquid is a fluid — something that flows easily when poured — although gases can also be called fluid. When your doctor told you to drink lots of fluids to help your cold symptoms, she probably meant things like water or orange juice, not root beer floats. If a movement is smooth, it can also be described as fluid. You have to make fluid arm movements in ballet so you look graceful.

Example Sentences

He died of heart failure and acute pulmonary edema -- a condition where fluid accumulates in the lungs -- two weeks after going under the knife.

From Barron's • Apr. 14, 2026

We’re in a fluid, dangerous story that isn’t going away any time soon.

From The Wall Street Journal • Apr. 9, 2026

In this new work, the team captured real-time fluid movement deep within the brain's lymphatic structures.

From Science Daily • Apr. 9, 2026

Her husband took her to hospital, where doctors performed a CT scan which picked up free fluid in the abdominal area.

From BBC • Apr. 8, 2026

They swayed as they sang, and Olanna imagined that the mango and gmelina trees swayed too, in agreement, in one fluid arc.

From "Half of a Yellow Sun" by Chimamanda Ngozi Adichie

Related Words

• flexible
• shifting
• unsettled
• unstable