fluid

UK: /ˈfluːɪd/ US: /ˈfluɪd/ ,(flo̅o̅′id)

WordReference Random House Learner's Dictionary of American English © 2026

flu•id /ˈfluɪd/   n. 

1. Hydraulics, Physics a substance, such as a liquid or gas, that is capable of flowing and that changes its shape when acted upon by a force:
   [uncountable] Fluid dripped from the tank.
   [countable] to drink fluids.

adj. 

2. Hydraulics flowing or capable of flowing:
   a fluid substance.
3. changing easily or readily; not fixed, stable, or rigid:
   Our plans are fluid.
4. smooth and flowing:
   a dancer with fluid gestures.
5. Business convertible into cash;
   liquid:
   fluid assets.

flu•id•i•ty /fluˈɪdɪti/  n. [uncountable]
flu•id•ly, adv. See -flu-.

WordReference Random House Unabridged Dictionary of American English © 2026

flu•id  (flo̅o̅′id), n. 

1. Hydraulics, Physics 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.

adj. 

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

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

flu′id•al, adj. 
flu′id•ly, flu′id•al•ly, adv. 
flu′id•ness, n. 
Synonyms

• 2. See liquid. 

Collins Concise English Dictionary © HarperCollins Publishers::

fluid / 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

Etymology: 15th Century: from Latin fluidus, from fluere to flow

ˈfluidness noun