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The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by ${\displaystyle k}$, ${\displaystyle \lambda }$, or ${\displaystyle \kappa }$ and, in SI units, is measured in W·m⁻¹·K⁻¹. It quantifies the proportionality between the heat flux (heat flow rate per unit area, W·m⁻²) and the temperature gradient (K·m⁻¹) in the direction of heat transport. The reciprocal of thermal conductivity is called thermal resistivity.

Materials with high thermal conductivity transfer heat more efficiently than those with low thermal conductivity. Heat transport can arise from different microscopic mechanisms: In metals, thermal conductivity is typically dominated by free electrons, whereas in dielectric materials such as diamond it is largely due to lattice vibrations. Materials with high thermal conductivity are used in heat sink applications, while materials with low thermal conductivity, such as mineral wool or Styrofoam, are used for thermal insulation.

The defining equation for thermal conductivity is ${\displaystyle \mathbf {q} =-k\nabla T}$, where ${\displaystyle \mathbf {q} }$ is the heat flux, ${\displaystyle k}$ is the thermal conductivity, and ${\displaystyle \nabla T}$ is the temperature gradient. This is known as Fourier's law for heat conduction. Although ${\displaystyle k}$ is commonly treated as a scalar, it is a second-rank tensor in the general case. The tensorial description is necessary for anisotropic materials.