Sometimes creation of a new term has a way of crystallizing a new concept. I hope that "mesogrid" will work that way. I envision mesogrids as evolutionary steps towards a supergrid. I give Steve Eckroad credit for the mesogrid concept, though it was my idea to introduce the term. This figure from Steve's US Patent 7,518,266 (assigned to EPRI) illustrates the concept:
Essentially, a mesogrid is a DC grid that connects into an AC grid at several AC nodes (major transformer yards in the above sketch) and so has both transmission and distribution functions. Steve envisioned an HVDC loop around a city as a way to improve reliability and resilience against rolling blackouts. Mesogrids are differentiated from microgrids because of connecting to the AC grid at several points, and are smaller and lower in operating voltage compared to future supergrids. This makes them very well suited as intermediate steps towards supergrids.