The cooling of a liquid to a temperature lower than its normal freezing temperature is called supercooling.
A supercooled cloud is containing pure water droplets at temperatures considerably below the nominal freezing temperature of 0°C. With very pure water (i.e. free from pollutants), supercooling of liquid drops can occur down to around minus 40°C. Altocumulus clouds, for example, are usually composed of water droplets at temperatures well below zero.