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Any liquid or gas that cannot sustain a shearing force when at rest and that undergoes a continuous change in shape (see flow) when subjected to such a stress. Compressed fluids exert an outward pressure that is perpendicular to the walls of their containers. A perfect fluid lacks viscosity, but real fluids do not.
This entry comes from Encyclopædia Britannica Concise. For the full entry on fluid, visit Britannica.com.