fluid

Pluralfluids

1. 1
   nouncountable or uncountable

   A liquid substance, as distinct from a solid or a gas.

   Brake fluid leaked slowly from the cracked line.

   The container was filled with a clear, odorless fluid.

   Similarliquid

   Oppositesolid

   Note

   Everyday and clinical speech narrows the word to liquids alone. That is why a clinician can ask whether a lump is solid, fluid or gas, a set of three that would collapse under the physicist's broader meaning. The plural is the household idiom: drink plenty of fluids means anything drinkable. The Doctor in Star Trek: Voyager (1995) makes a running joke of exactly that advice.

2. 2
   nouncountable

   Any substance that flows readily and takes the shape of its container: a liquid, gas, or plasma.

   Engineers study how a fluid behaves when forced through narrow pipes.

   Both gases and liquids count as fluids in physics.

   Oppositesolid

   Note

   The physicist's term is broad enough to cover air and plasma as well as water. A fluid yields and flows instead of holding a shape of its own. A vortex is a spinning mass of fluid, whether it is water going down a drain or a smoke ring. The word comes through Latin fluidus, 'flowing', from fluō, 'to flow'. Despite appearances, it is not related to English flow, a native word from a different root.

3. 3
   adjective

   In a changeable state; subject to ongoing change rather than fixed.

   The political situation remained fluid for weeks after the election.

   Negotiations were fluid, with the terms shifting almost daily.

   Similarchangeable, unstable, volatile, unsettled, fluctuating

   Oppositestable, fixed, settled, static

4. 4
   adjective

   Moving smoothly and gracefully, or seeming to flow like a liquid in motion.

   The gymnast performed a fluid sequence without a single pause.

   Her handwriting is fluid, each letter flowing into the next.

   Similarsmooth, graceful, flowing, fluent, elegant

   Oppositejerky, stiff, awkward, clumsy

5. 5
   adjective

   Relating to a fluid or to the movement and behavior of fluids.

   The course covers fluid dynamics and the forces involved.

   Their research focuses on fluid resistance in narrow channels.

   Note

   This is the classifying adjective, which sits before a noun to name a field or property: fluid dynamics, fluid resistance. It does not grade, so nothing is more fluid dynamics than something else. That sets it apart from the 'smooth-moving' adjective, which happily takes more and most. If you can put very in front of it, you are using a different sense.

6. 6
   adjective

   Able to be converted into cash readily.

   The company kept most of its assets fluid to cover sudden costs.

   Investors preferred fluid holdings during the uncertain market.

   Similarliquid

   Oppositeilliquid

7. 7
   nouncolloquialusually plural

   Liquid delivered directly into a patient's vein as part of medical treatment.

   After surgery she was put on fluids for two days.

   The ER doctor ordered a fluid bolus for the dehydrated marathon runner.

   Note

   This is hospital shorthand, and it is nearly always plural: a patient is on fluids, and staff start, run or stop them. The singular survives mostly before another noun, as in a fluid bolus. The setting carries the meaning. On a ward, fluids are what runs through the drip, not the everyday liquids a patient is told to drink. Because the usage is colloquial, careful writing is safer with intravenous fluids in full.

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