In physics, a fluid is a liquid, gas, or other material that may continuously move and deform (flow) under an applied shear stress, or external force.
Although the term fluid generally includes both the liquid and gas phases, its definition varies among branches of science.
[2] Non-Newtonian fluids like Silly Putty appear to behave similar to a solid when a sudden force is applied.
In particle physics, the concept is extended to include fluidic matters other than liquids or gases.
A consequence of this behavior is Pascal's law which describes the role of pressure in characterizing a fluid's state.