A supershell is a very large shell, i.e., a very large portion of the interstellar medium (ISM) that surrounds some volume, the shell having some material distinct from that within and that around it. The word superbubble may also be used, and both terms may sometimes be meant to include the material within the cavity as well (but I'll use shell here to indicate the exterior surrounding layers). An HI supershell is a supershell whose material (surrounding the cavity) is neutral atomic hydrogen, constituting significant HI regions, Supershells can be large enough to protrude out of the galactic disk. This can emit gas from the disk, termed a galactic fountain and feed a cloud of hot gas beyond the disk but within the galactic halo, termed the galactic corona or gaseous corona.
Shells seem to be driven by some kind of energetic event, but some supershells show so much excess energy that a single supernova is very unlikely to be sufficient.