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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Synonym Usage

See liquid.

Other Word Forms

Derived Forms

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

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

The study, published in Journal of Fluid Mechanics Rapids, updates a formula that is more than 100 years old and addresses a major gap in aerosol science.

From Science Daily • Feb. 8, 2026

Because of its rarity and high fatality rate, the Amniotic Fluid Embolism Foundation says it’s exceptionally difficult to study.

From Los Angeles Times • Apr. 12, 2025

Fluid injection can also expand rock, in a mechanism called poroelastic stress.

From Science Daily • May 8, 2024

Fluid intake is vital, especially if the weather is warmer and the fasting person is planning strenuous exercise.

From Salon • Mar. 11, 2024

There was another public-house just beyond the Sarcey's Fluid advertisement.

From In Accordance with the Evidence by Onions, Oliver [pseud.]

Related Words

• flexible
• shifting
• unsettled
• unstable