fluid

American  

[floo-id] / ˈflu ɪd /

noun

• fluids
  plural

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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Synonym Usage

See liquid.

Other Word Forms

Derived Forms

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

Inflected Forms

Nouns

• fluids
  plural

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

See Examples For:

fluid fluids

"This will enable us to study the properties and behavior of this exotic fluid in unprecedented detail."

From Science Daily ● Oct. 6, 2026

I spent 25 days in the hospital to drain the peritonitis fluid and a year in bed at home healing.

From The Wall Street Journal ● Oct. 6, 2026

She developed pleural effusion — fluid between the lungs and ribs — compressing her lungs and making it hard for her to breathe.

From MarketWatch ● Oct. 5, 2026

The Navier-Stokes problem, which deals with complex fluid dynamics, had withstood the assaults of mathematicians for almost two centuries.

From The Wall Street Journal ● Oct. 2, 2026

“Aha,” the mysterious potion brewer says, lifting a vial of blue fluid in one hand.

From "The Sea in Winter" by Christine Day

“Drink plenty of fluids, stay in an air-conditioned room, stay out of the sun, and check up on relatives and neighbors,” the weather service advises.

From Los Angeles Times ● Oct. 1, 2026

During the marathon, he has been eating lightly and drinking fluids while taking the permitted five minutes of rest for every continuous hour of activity.

From BBC ● Sep. 28, 2026

One of these organisms is the deep-sea archaeon Methanocaldococcus infernus, which lives in volcanic marine environments where vent fluids can reach temperatures above the boiling point of water.

From Science Daily ● Sep. 19, 2026

The company sells chemicals used in energy production called completion fluids, mainly to deepwater producers in the North Sea and off the coast of Brazil and the U.S.

From MarketWatch ● Sep. 19, 2026

Baba sets it up with a syringe to get fluids into the boy.

From "Without Refuge" by Jane Mitchell

Related Words

• flexible
• shifting
• unsettled
• unstable