The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by , , or and in SI units is measured in W·m·K. In such units, it is the amount of thermal power in watts (joules per second) that flows from a hotter region to a colder region per Kelvin (or degree Celsius) difference in temperature per meter of separation.
Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal conductivity. For instance, metals typically have high thermal conductivity and are very efficient at conducting heat, while the opposite is true for insulating materials such as mineral wool or Styrofoam. Metals have this high thermal conductivity due to free electrons facilitating heat transfer. Correspondingly, materials of high thermal conductivity are widely used in heat sink applications, and materials of low thermal conductivity are used as thermal insulation. The reciprocal of thermal conductivity is called thermal resistivity.