From AMS Glossary
A cloud containing both water drops (supercooled at temperatures below 0°C) and ice crystals, hence a cloud with a composition between that of a water cloud and that of an ice- crystal cloud.
A mixed cloud is unstable in the sense that the equilibrium vapor pressure difference between water drops and ice crystals at subfreezing temperatures promotes growth of the ice crystals at the expense of the water drops, which is the basis for the Bergeron–Findeisen theory of precipitation. Most convective clouds extending into air colder than about -10°C are mixed clouds, though the proportion of ice crystals to water drops may be small until the cloud builds to levels of still lower temperature. Some clouds (lenticularis, mountain wave clouds) form at temperatures near -35°C and contain only very small amounts of ice crystals.