= Pressure tensor
{title2=$P_{ij}$}
In the local rest frame of a gas, the <pressure tensor> is the directional <momentum flux> $P_{ij}=\int p_iv_jf(\mathbf p)\,d^3p$, where $f$ is the <phase-space distribution function> per unit <momentum> volume and $\mathbf v$ is particle velocity. For an isotropic distribution, $P_{ij}=P\delta_{ij}$, recovering the <kinetic pressure of an isotropic gas>. An anisotropic distribution can exert different normal <pressures> along different directions.
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