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.

---

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

---

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

---

fluid Cultural  

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

---

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 gene helps control the movement of fluid through the brain, a function that supports the organ's natural waste-clearing system.

From Science Daily ● Jul. 31, 2026

This can lead to severely painful episodes, which experts say can sometimes be prevented by wrapping up warm and drinking plenty of fluid, and daily antibiotics can reduce risk of infections.

From BBC ● Jul. 23, 2026

There are people out there who don’t know the exact amount of their income, especially if it’s fluid.

From MarketWatch ● Jul. 22, 2026

Katzenback said the airy feeling I had leaving my treatment was likely linked to temporary changes in fluid distribution, which can “make people feel less swollen or heavy, especially after travel.”

From The Wall Street Journal ● Jul. 21, 2026

Land, not fluid forms of capital, was their ultimate measure of wealth.

From "Founding Brothers: The Revolutionary Generation" by Joseph J. Ellis

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

From Science Daily ● Aug. 8, 2026

Unidentified ashes in mouldy boxes and excrement and bodily fluids on the floor of the Hessle Road premises were among the other disturbing discoveries.

From BBC ● Jul. 31, 2026

Elizabeth Hagerty told prison doctors she had heat rash covering her body and couldn’t keep fluids down; they sent her back to her cell, and she died two days later, a month short of release.

From Slate ● Jul. 31, 2026

A viral haemorrhagic fever, Ebola spreads through close contact and infected bodily fluids.

From Barron's ● Jul. 30, 2026

Paramedics loaded Sasha onto a gurney and hooked up an IV. Warm fluids flowed into Sasha’s veins.

From "The 57 Bus" by Dashka Slater

Related Words

• flexible
• shifting
• unsettled
• unstable