fluid
gb/flˈuɪd/ us/flˈuɪd/
B2
English
~ 5100
Senses
0 senses
Noun
B2
entity
1
Substance like a liquid that flows
A substance, usually a liquid, that can move and change shape easily and is not solid.
Usage pattern:
a fluid flows through [something]
Common mistake:
Learners sometimes think 'fluid' only means 'gas', but in everyday English it usually means a liquid that flows, like water or oil. They also sometimes confuse 'fluid' with 'fluent'; 'fluent' is used for language ability, not for substances.
Synonyms
liquid (Almost the same meaning; 'liquid' is more common in everyday speech, while 'fluid' can sound more formal or technical.)
Antonyms
solid (A solid keeps its shape and does not flow, which is the opposite of a fluid.)
Collocations
a clear fluid
|thick fluid
|fluid flows through [something]
|cooling fluid
|fluid level
|fluid intake
Themes
science
|daily life
|core_vocabulary
Examples
- The mechanic noticed a strange fluid leaking from the engine.
- Blood is a fluid that carries oxygen around the body.
- Make sure the cooling fluid in the system is topped up before you start the machine.
- Scientists collected samples of the fluid for further analysis.
- The pipe burst and sprayed a dark, oily fluid across the floor.
Learner examples
- A fluid can move and change its shape.
- Water is a common fluid in our daily lives.
- The doctor took some clear fluid from my knee.
- The car lost a lot of fluid after the accident.
adjective
B2
property
2
Likely to change; not fixed
Changing or easy to change; not fixed or firm.
Usage pattern:
[something] is fluid / fluid [situation/plan/identity]
Common mistake:
Learners sometimes use 'fluid' instead of 'fluent' to talk about language ability (e.g. 'He is fluid in English'). The correct word for language ability is 'fluent'. Use 'fluid' here only for things like plans, situations, or identities that can change.
Synonyms
flexible (Suggests something can be easily changed by choice; 'fluid' stresses that things are changing or unstable by nature.)
Antonyms
fixed (Something fixed is firm and not expected to change, which is the opposite of fluid in this sense.)
Collocations
a fluid situation
|fluid plans
|fluid identity
|fluid schedule
|fluid market
|remain fluid
Themes
business
|society
|core_vocabulary
Examples
- Our travel plans are still fluid, so we can leave a day earlier if you need to.
- The political situation in the region remains fluid and unpredictable.
- In a fluid job market, people often change careers several times.
- Definitions of family are increasingly fluid in modern society.
- Project deadlines are fluid at this stage, depending on the client's feedback.
Learner examples
- The schedule is fluid, so the time may change.
- The situation is fluid and can change quickly.
- Our plans are fluid, so nothing is sure yet.
- Her role in the company is fluid and not clearly defined.
adjective
B2
property
3
Smooth, flowing, graceful
Smooth and easy, like flowing water, especially when talking about movement, style, or design.
Usage pattern:
fluid [movement/style/line]
Common mistake:
Learners sometimes confuse 'fluid' with 'fluent'. Use 'fluent' mainly for language ability (for example, 'fluent English'), and 'fluid' for movement, style, or design that is smooth and flowing.
Synonyms
smooth (Very close in meaning; 'fluid' often adds a sense of continuous, natural flow or grace.)
Antonyms
awkward (An awkward movement is stiff or clumsy, the opposite of fluid movement.)
Collocations
fluid motion
|fluid movement
|fluid style
|fluid lines
|fluid performance
|fluid transitions
Themes
art
|sports
|communication
|core_vocabulary
|arts_entertainment
Examples
- The dancer's fluid movements held the audience's attention.
- He has a fluid writing style that is easy to read.
- The car's fluid design makes it look fast even when it is parked.
- Her fingers moved in a fluid motion across the piano keys.
- The speaker used fluid transitions between topics, so the talk felt natural.
Learner examples
- The player made a fluid move and scored.
- She speaks with a fluid, natural voice.
- The artist drew long, fluid lines on the paper.
- His dancing is very fluid and beautiful.
Noun
C1
entity
4
Liquid from the body
A liquid from the body, like blood, urine, or saliva.
Usage pattern:
bodily fluids / body fluids
Common mistake:
Learners may not realize that 'fluids' in medical or health contexts almost always refers to body liquids, not just any liquid. In general descriptions, use 'liquid', but in health and medical warnings, 'bodily fluids' is the standard term.
Synonyms
bodily fluid (A more explicit medical term for liquids from the body; often used in the plural 'bodily fluids'.)
Antonyms
tissue (Body tissue is solid material of the body, contrasting with liquid body fluids.)
Collocations
bodily fluids
|body fluids
|infected fluids
|exchange of fluids
|exposure to fluids
|fluid samples
Themes
health
|medicine
|body
Examples
- Healthcare workers must avoid contact with patients' bodily fluids.
- The lab tested several body fluids for signs of infection.
- Blood and other fluids were found at the scene.
- The virus can be spread through the exchange of bodily fluids.
- Samples of spinal fluid were taken to check for disease.
Learner examples
- Doctors wear gloves to avoid touching body fluids.
- Blood is a body fluid.
- The nurse took a sample of fluid from his knee.
- Some diseases spread through bodily fluids.
Noun
technical
C1
entity
5
Substance that flows, liquid or gas
In science, a substance that can flow, including both liquids and gases.
Usage pattern:
a fluid flows through [a pipe/system]
Common mistake:
In everyday English, many people use 'fluid' to mean only liquids. In technical science and engineering, 'fluid' usually includes both liquids and gases, which can confuse learners who do not expect air to be called a 'fluid'.
Synonyms
liquid or gas (A descriptive phrase rather than a single word; it explains the technical meaning that both liquids and gases are treated as fluids in physics.)
Antonyms
solid (In physics, solids do not flow under small forces, unlike fluids.)
Collocations
fluid dynamics
|fluid mechanics
|viscous fluid
|incompressible fluid
|fluid flow
|fluid pressure
Themes
science
|engineering
|technology
Examples
- In physics, air is treated as a fluid when modeling airflow over a wing.
- Fluid mechanics studies how fluids move and interact with forces.
- Engineers must calculate how the fluid will flow through the new pipeline.
- The pump is designed for use with non-corrosive fluids only.
- The experiment compares the behavior of different viscous fluids.
Learner examples
- In science, water and air are both fluids.
- Fluid mechanics is the study of moving fluids.
- A fluid can change its shape when it moves.
- The engineer tested how the fluid flowed in the tube.
Grammar
Dictionary form
fluid
IPA
GB/flˈuɪd/
US/flˈuɪd/
Syllables & stress
flu - id (2 syllables)
Stress on 1st syllable (flu)
Word forms
Adjective
- Base: fluid
Noun
- Base: fluid
- Plural: fluid, fluids
Word family
Forms
fluids
noun
Core family
fluidity
noun
Related words
fluidly
adverb
Expressions
amniotic fluid
1/ˈæmniɑtɪk flˈuɪd/
fixed phrase
Liquid around a baby in the womb
The clear liquid in a pregnant woman's womb that surrounds and protects the baby.
Examples
- The doctor checked the level of amniotic fluid during the ultrasound.
- A sudden loss of amniotic fluid can be a medical emergency.
- The baby floats in amniotic fluid inside the uterus.
body fluids
1/bˈɑdi flˈuɪdz/
fixed phrase
Liquids inside the body
The liquids inside the body, such as blood, urine, and saliva.
Examples
- Nurses must wear gloves when they may touch body fluids.
- Some viruses are spread through contact with body fluids.
- The sample was tested for viruses in the body fluids.
brake fluid
1/bɹˈeɪk flˈuɪd/
fixed phrase
Liquid in a car's brakes
The liquid in a vehicle's brake system that helps the brakes work.
Examples
- You should check your brake fluid regularly.
- Low brake fluid can make it harder to stop the car.
- The garage said the brake fluid needed to be changed.
cerebrospinal fluid
1/sˈɛɹəbɹoʊspɪnəl flˈuɪd/
fixed phrase
technical
Protective liquid around brain and spine
The clear liquid that surrounds and protects the brain and spinal cord.
Examples
- Bacteria were found in the patient's cerebrospinal fluid.
- An increase in cerebrospinal fluid pressure may cause headaches.
- The test requires a small sample of cerebrospinal fluid.
fluid dynamics
1/flˈuɪd daɪnˈæmɪks/
fixed phrase
technical
Study of moving fluids
A part of physics that studies how liquids and gases move.
Examples
- She is doing a PhD in fluid dynamics.
- Fluid dynamics helps engineers design better airplane wings.
- The course covers basic fluid dynamics and heat transfer.
transmission fluid
1/tɹænzmˈɪʃən flˈuɪd/
fixed phrase
Liquid for a car's transmission
The liquid that helps a car's transmission work smoothly and stay cool.
Examples
- The transmission fluid should be changed every 60,000 miles.
- Low transmission fluid can damage the gearbox.
- He checked the transmission fluid level before the long trip.
gender fluid
1/d͡ʒˈɛndɚ flˈuɪd/
fixed phrase
changing gender identity over time
When a person's idea of their gender changes over time or in different situations.
Examples
- They describe themselves as gender fluid.
- They describe themself as gender fluid rather than male or female.
- Being gender fluid means your identity can shift between male, female, or other genders.
- Some gender fluid people feel more masculine on some days and more feminine on others.
- The article explains what it means to be gender fluid.
hydraulic fluid
1/haɪdɹˈɔlɪk flˈuɪd/
fixed phrase
technical
special liquid for hydraulic systems
The special liquid that flows inside a hydraulic system to carry force and make parts move.
Examples
- The mechanic topped up the hydraulic fluid in the lift.
- Low hydraulic fluid can cause the steering to feel heavy and unresponsive.
- Low hydraulic fluid can cause the system to fail.
- Always use the recommended hydraulic fluid for your tractor.
- Always use the correct hydraulic fluid for this machine.
- The leak was traced to a worn seal that was letting hydraulic fluid escape.
spinal fluid
1/spˈaɪnəl flˈuɪd/
fixed phrase
Liquid around brain and spine
The clear liquid around the brain and spinal cord.
Examples
- They took a sample of spinal fluid to test for infection.
- The disease can be detected in the spinal fluid.
- Pressure in the spinal fluid was higher than normal.
spinal fluid
2/spˈaɪnəl flˈuɪd/
fixed phrase
technical
fluid around brain and spine
The clear liquid around the brain and spinal cord that protects them.
Examples
- They tested her spinal fluid for signs of infection.
- A leak of spinal fluid can cause severe headaches.
- The sample of spinal fluid showed no cancer cells.