From AMS Glossary
Convective storm that contains similar radar characteristics to those of a supercell (e.g., hook echo, WER, BWER), but is significantly smaller in height and width.
The diameter of the radar-detected rotation is 1–8 km. This is a relatively new storm type, the existence of which has been confirmed by data from the recently installed WSR–88D radars in the United States. Mini-supercells occur in areas where the height of the equilibrium level is low, most often in the northern United States, but possibly under certain weather conditions in any area of the world. They are sometimes found in landfalling tropical cyclones.