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.

---

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.

---

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; -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

This slower pattern suggested that the fluid was part of the lymphatic system rather than the circulatory system.

From Science Daily • Apr. 9, 2026

Simmer gently until thick enough to coat the back of a spoon but still fluid, 3–5 minutes.

From Salon • Apr. 7, 2026

The company is addressing an age-old problem in enterprise technology: the multitude of siloed systems and data sources that don’t communicate with one another in a fluid way.

From The Wall Street Journal • Apr. 3, 2026

Olise, 24, is flourishing in a fluid Bayern attack.

From BBC • Mar. 31, 2026

“I want to go with you,” he wailed, wiping away the eye fluid.

From "Klawde: Evil Alien Warlord Cat" by Johnny Marciano and Emily Chenoweth

Related Words

• flexible
• shifting
• unsettled
• unstable