From AMS Glossary
A relatively cool layer of air, usually adjacent to a ground surface cooled by net loss of radiation, in which the air temperature increases with height.
Sometimes this term is also loosely extended to include layers in which the potential temperature increases with height, that is, to all statically stable layers of air formed by radiatively cooled ground. Radiational cooling is responsible for forming nocturnal stable boundary layers. It is also associated with formation of dew, frost, and fog if the humidity is sufficiently high. It is rare for elevated radiation inversions to form because cooling that creates a relative minimum in potential temperature at some height above the surface would cause cold air to descend from the inversion, creating turbulence that would destroy the inversion.