English Reference
noun · adjective

fluid

n. · adj.
US /ˈfɫuəd/ UK /flˈuːɪd/ flu·id UK spelling fluids

  1. 01 B2 Upper Intermediate Technical Science

    n.any substance that flows easily and takes the shape of its container, such as water or air.

    n.a substance, such as a liquid, gas, or plasma, that deforms under shear stress and conforms to the shape of its container.

    Simple

    The mechanic checked the levels of the brake fluid in the car.

    In context

    The laboratory equipment was designed to measure the viscosity of various industrial fluids under extreme pressure conditions.

  2. 02 C1 Advanced Technical Medicine

    n.liquids given to a patient, often through a tube in the arm.

    n.intravenous solutions administered to a patient to maintain hydration or deliver medication.

    Simple

    The doctor said the patient needed more fluids to recover from the fever.

    In context

    The nursing staff monitored the administration of intravenous fluids to ensure the patient remained stable after the surgery.

  3. 03 B2 Upper Intermediate

    adj.likely to change; not fixed or settled.

    adj.subject to change or transition; characterized by a lack of fixed structure.

    Simple

    The travel plans are still fluid because we haven't booked the flights yet.

    In context

    The political situation remained fluid as negotiators struggled to reach a consensus before the midnight deadline.

  4. 04 B2 Upper Intermediate

    adj.moving in a smooth and beautiful way.

    adj.characterized by a smooth, elegant, and continuous quality of motion.

    Simple

    The dancer's movements were fluid and graceful.

    In context

    The athlete's fluid running style allowed him to maintain a high pace without appearing to exert significant effort.

  5. 05 C1 Advanced FormalTechnical

    adj.describes money or assets that can be used or moved easily.

    adj.referring to assets that are easily converted into cash without significant loss of value.

    Simple

    The company needs to keep some fluid assets to pay for unexpected repairs.

    In context

    The investment firm advised the client to maintain a portion of their portfolio in fluid capital to respond to market volatility.

    illiquidfrozen

Ebola is rare but highly contagious and can be contracted from body fluids such as vomit, blood or semen, including from dead bodies, and from contaminated surfaces and materials such as bedding and clothing.

The Guardian· 13 Aug 2026

Six to eight glasses of fluid a day is normally recommended by the NHS, , external and in a heatwave you could need a lot more.

BBC News· 8 Jul 2026

From Middle English fluid, from Latin fluidus (“flowing; fluid”), from Latin fluō (“to flow”), from Proto-Indo-European bʰleh₁- (“to swell; surge; overflow; run”). Akin to Ancient Greek φλύειν (phlúein, “to swell; overflow”). Not related to English flow, which is a native, inherited word from plew-, but is distantly related from English bleat.

Etymology adapted from Wiktionary, available under CC BY-SA 4.0.

This entry
Origin ine-pro *bʰleh₁-
Level C1 · Advanced
Frequency Common
Updated