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.
pertaining to a substance that easily changes its shape; capable of flowing.
consisting of or pertaining to fluids.
changing readily; shifting; not fixed, stable, or rigid: fluid movements.
convertible into cash: fluid assets.

1595–1605; < Latin fluidus, equivalent to flu(ere) to flow + -idus -id4

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

fluid, gas, liquid (see synonym study at liquid).

2. See liquid.
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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
fluid (ˈfluːɪd)
1.  a substance, such as a liquid or gas, that can flow, has no fixed shape, and offers little resistance to an external stress
2.  capable of flowing and easily changing shape
3.  of, concerned with, or using a fluid or fluids
4.  constantly changing or apt to change
5.  smooth in shape or movement; flowing
[C15: from Latin fluidus, from fluere to flow]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

c.1600, from L. fluidus "fluid, flowing," from fluere "to flow" (see fluent). The noun is 1660s, from the adj.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

fluid flu·id (flōō'ĭd)
An amorphous substance whose molecules move freely past one another; a liquid or gas. adj.
Of or characteristic of a fluid.

flu·id'i·ty (-ĭd'ĭ-tē) or flu'id·ness n.
The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
fluid  [%PREMIUM_LINK%]     (fl'ĭd)  Pronunciation Key 
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. See also state of matter, viscosity.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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American Heritage
Cultural Dictionary

fluid definition

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

The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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