Skin temperature is defined as the temperature of the surface at radiative equilibrium. It forms the interface between soil, snow or ice and the atmosphere. Units are Kelvin (K).
In ERA-Interim and ERA5 skin temperature (skt, code 235) is derived from the surface energy balance.
For ERA-Interim, the calculation of skin temperature is described in the ECMWF IFS model documentation, cycle 31r2, Part IV: Physical Processes. Note that in ERA-Interim, over ocean, the skin temperature is set to the surface temperature.
For ERA5, the calculation of skin temperature is described in the ECMWF IFS model documentation, cycle 41r2, Part IV: Physical Processes.