From AMS Glossary
A cloud formed when two subsaturated volumes of moist air with different temperatures and vapor pressures mix isobarically and adiabatically to form a volume of moist air with an intermediate temperature and vapor pressure above the saturation value at that temperature.
Popular but misleading terms for mixing clouds are steam, steam fog, and steam clouds. These terms obscure the essential mechanism by which mixing clouds are formed and confuse water vapor with liquid water. Mixing clouds are the consequence of neither cooling nor heating because both volumes enter into the mixture symmetrically. They are a consequence of the nonlinearity of the Clausius–Clapeyron equation together with the linearity of the first law of thermodynamics for moist air. An example of a mixing cloud is a condensation trail.