Mesocyclone

A storm-scale region of rotation, typically around 2-6 miles in diameter and often found in the right rear flank of a supercell. The circulation of a mesocyclone covers an area much larger than the tornado that may develop within it.

Properly used, mesocyclone is a radar term; it is defined as a rotation signature appearing on Doppler radar that meets specific criteria for magnitude, vertical depth, and duration. Therefore, a mesocyclone should not be considered a visually-observable phenomenon.