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

If the data were collected at different times, for example, some of the information -- especially on a fluid process like gaining US citizenship -- could be out-of-date, he explained.

From Barron's ● Aug. 20, 2026

It was a punch delivered in Atlanta on Sunday and felt all the way in Washington, smooth and fluid, like one of Sugar Ray Robinson’s left hooks or Muhammad Ali’s left jabs.

From Salon ● Aug. 20, 2026

The front four play in a very fluid manner with the four defenders and two central midfielders taking up more consistent positions behind them.

From BBC ● Aug. 19, 2026

The Middle East situation remains fluid, with little progress in talks to reopen the Strait of Hormuz.

From The Wall Street Journal ● Aug. 17, 2026

It's a fluid movement, one that says she's done it a million times, that in this meeting everybody knows their place.

From "Girl in the Blue Coat" by Monica Hesse

St. John Bosco began its preparation to be hydrated and avoid cramps on Monday with constant reminders on taking fluids all week by athletic trainer Daisy Martinez.

From Los Angeles Times ● Aug. 22, 2026

The virus, which spreads through contact with bodily fluids and causes a haemorrhagic fever, "is moving faster than any previous Ebola outbreak", he warned.

From Barron's ● Aug. 18, 2026

Briny fluids likely helped drive at least some of that chemistry.

From Science Daily ● Aug. 8, 2026

During this hormonal shift, your body holds on to more fluids, meaning you may end up with a puffier face, general bloating and tender, swollen breasts.

From BBC ● Aug. 3, 2026

“The electrical current travels up the fluids in your brain, turning your head into an antenna.”

From "Dry" by Neal Shusterman and Jarrod Shusterman

Related Words

• flexible
• shifting
• unsettled
• unstable