Difference between fluids and nonfluids

All liquids are fluids but not all fluids are liquids. Fluids basically describe anything that can flow as a result of a difference in pressure between two points. Liquids on the other hand are a subset of fluids. Liquids are the incompressible fluids. These are the fluids who's density does not change sharply with pressure.

Gases are also fluids, they form the compressible fluids. They do flow as a result of difference in pressure between two points but at the same time they can be compressed or expanded. Their density fluctuates with pressure.

Hence fluids consist of 2 mutually exclusive groups of compressible and incompressible fluids. Liquids are the incompressible fluids and hence form a subset of the more general term fluid.

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