= Thermal expansion
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Thermal expansion is a change in a body's dimensions as its <temperature> changes under a specified mechanical constraint. A heated <gas> under a freely moving loaded piston usually increases its <volume (thermodynamics)> at fixed <pressure>; heating the same <gas> in a rigid vessel instead raises its <pressure>. The <coefficient of thermal expansion> quantifies the first response. Expansion can be negative, so the name does not assert that every substance grows on heating.
For an <ideal gas> at fixed <pressure> and particle number, $V=Nk_BT/P$ gives $V_2/V_1=T_2/T_1$. Heating from $300\,\mathrm K$ to $330\,\mathrm K$ therefore increases the <volume (thermodynamics)> by exactly $10\%$ within the <ideal gas> model. For a <solid>, specify whether the measurement concerns length or <volume (thermodynamics)>: for uniform expansion in all three directions, the volumetric coefficient is three times the <linear thermal expansion coefficient>, because $V\propto L^3$. Direction-dependent expansion requires the separate dimensional responses. https://ocw.mit.edu/courses/3-20-materials-at-equilibrium-sma-5111-fall-2003/e402e3f0f555e15ff3af29b5b44b4e57_notes1.pdf[G. Ceder, MIT thermodynamics notes, section 1.14: linear and volumetric expansion]{ref}
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