The defining equation for thermal conductivity is q = − − k ∇ ∇ T , where q is the heat flux, k is the thermal conductivity, and ∇ ∇ T is the temperature gradient. This is known as Fourier's Law for heat conduction. Although commonly expressed as a scalar, the most general form of thermal conductivity is a second-rank tensor. However, the tensorial description only becomes necessary in materials which are anisotropic.