The rotating region within a supercell that is typically about 2 to 6 miles wide is a mesocyclone.
A mesocyclone is a storm-scale vortex of rising, rotating air inside a powerful thunderstorm, especially a supercell. Meteorologists usually identify it with Doppler radar, where rotation can appear as winds moving toward and away from the radar in a paired pattern. In the Northern Hemisphere, it commonly occupies the supercell’s right-rear flank, though its position varies with storm structure.
A mesocyclone is much larger than a tornado and does not automatically produce one. Tornadoes can form beneath or within a mesocyclone when smaller-scale rotation near the ground tightens and intensifies. That is why radar rotation alone is not proof that a tornado is on the ground; forecasters also look for debris signatures, storm evolution and observations from the surface.
The word “meso” refers to the mesoscale, a middle range between individual weather features and large synoptic systems. Mesocyclones generally last far longer than tornadoes and are important clues in identifying sustained, rotating supercells.