The heat equation states that the temperature T(x,t) in a uniform, solid medium satisfies the parabolic PDE
where κ denotes the thermal diffusivity of the medium. In more than one spatial dimension the second derivative is replaced by the Laplacian ∇2T, and if κ is not constant, then the RHS is replaced with div(κ∇T). The equation models diffusion more generally, for example that of particles, and so is often referred to as the diffusion equation.