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  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.
  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.

Origin of fluid

1595–1605; < Latin fluidus, equivalent to flu(ere) to flow + -idus -id4
Related formsflu·id·al, adjectiveflu·id·ly, flu·id·al·ly, adverbflu·id·ness, nounnon·flu·id, nounnon·flu·id·ly, adverbun·flu·id, adjective
Can be confusedfluid gas liquid (see synonym study at liquid)

Synonyms for fluid

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2. See liquid.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

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British Dictionary definitions for fluid


  1. a substance, such as a liquid or gas, that can flow, has no fixed shape, and offers little resistance to an external stress
  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
Derived Formsfluidal, adjectivefluidness, nounfluidly or fluidally, adverb

Word Origin for fluid

C15: from Latin fluidus, from fluere to flow
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for fluid

early 15c., from Middle French fluide (14c.) and directly from Latin fluidus "fluid, flowing, moist," from fluere "to flow" (see fluent). Figurative use from 1640s. Related: Fluidly.


1660s, from fluid (adj.).

Online Etymology Dictionary, © 2010 Douglas Harper

fluid in Medicine


  1. An amorphous substance whose molecules move freely past one another; a liquid or gas.
  1. Of or characteristic of a fluid.
Related formsflu•idi•ty (-ĭdĭ-tē) null n.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

fluid in Science


  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. See also state of matter viscosity.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

fluid in Culture


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

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