Rate of thermal energy travel through a material per unit thickness and unit temperature difference. Uses units of power (energy per unit time) per distance (specimen thickness) per temperature (temperature difference).
As with electrical conductivity, the inverse of thermal conductivity is thermal resistivity — but that property is seldom used.
ASTM testing standards include E1952 (using a differential scanner calorimeter) and E2584 (using a thermal capacitance calorimeter). ISO standards include 8894 (refractories) and 22007 (plastics).
Thermal conductivity has a more complicated relationship with temperature than most other thermal properties. Depending on the material, conductivity may either rise or fall with increasing temperature. Rising is more common, however.
For very pure materials, thermal conductivity can peak drastically near absolute zero. Pure aluminum can reach 6000 W/m-K around 10 degrees Kelvin, while copper goes up to 4000.