Sleet (often called wintry mix or wintry shower) is a form of precipitation which is often described during a storm with both snow and rain falling, or as a substance which is neither snow or rain. During flurries, snow can turn to sleet before turning into rain, and back into sleet before a return to falling as snow again.
Sleet typically occurs at a temperature at or just above freezing level (32°F/0°C). Sleet can also occur below freezing level, often during high humidity levels which usually causes for more wet snow. Equally, snow can fall at a temperature which would normally see sleet or rain if the humidity levels are low. Also, if the dewpoint is above freezing when the temperature is at or below freezing, snow can partially melt into sleet or rain. Again, a lower dewpoint below freezing at a temperature above freezing could cause for a more wintry mix of rain.
The METAR code is RASN.