= Thermal diffusion
Here <thermal diffusion> means the spreading of <temperature> by <thermal conduction>. In a stationary homogeneous material with constant <thermal conductivity> $k$, <mass density> $\rho$ and <specific heat capacity> $c$, energy balance and <Fourier's law> give the <heat equation> $\partial_tT=\alpha\nabla^2T$, where $\alpha=k/(\rho c)$ is the <thermal diffusivity>.
A disturbance spreads over a length of order $\sqrt{\alpha t}$, so equilibration across thickness $L$ takes a time of order $L^2/\alpha$. This is the heat-transport sense of the term; temperature-gradient-driven separation of components in a mixture is a distinct transport effect.
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