= Solution
Here an <elastic fluid> means a <gas> or vapour. Such a fluid is mobile, occupies the accessible space of its container, and is appreciably <compressible>. Its <pressure> acts normally on a confining surface. In static equilibrium the stress at a point is isotropic: a fluid has no equilibrium resistance to a sustained shear deformation. Pressure can still vary with height through <hydrostatic equilibrium>; isotropy does not mean the same pressure at every position.
Reducing a confined gas's <volume> at fixed <temperature> normally raises its pressure. Heating at fixed volume normally raises the pressure, and gases exchange material through <diffusion>. Their particles are in continual thermal motion, whose momentum transfer gives a microscopic account of gas pressure in the <kinetic theory of gases>.
For a dilute gas away from condensation, the approximate <ideal gas law> is
$$
\boxed{pV=nRT.}
$$
It contains <Boyle's law> at fixed temperature and the corresponding temperature-expansion laws. These simple proportionalities are approximations, not exact properties of every elastic fluid at every density. Intermolecular interactions matter in a dense gas, and a vapour can <condense>; the classification does not promise that every sample remains gaseous under arbitrary compression or cooling.
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