There is continuity between the gaseous and liquid state. The term fluid is used for either liquid or a gas to recognize this continuity. A liquid can be viewed as a very dense gas. Liquid and gas can be distinguished only when the fluid is below its critical temperature and its pressure and volume lie under the dome (in isotherm) since in that situation liquid and gas are in equilibrium and a surface separating the two phases is visible.
Chapter: States of Matter
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