M.S.O. (MULTIPLE SYSTEM OPERATOR) An MSO is a Cable TV operator of multiple cable systems. A cable system, by FCC definition, is a facility serving a single community or a distinct governmental entity. Also note that each town has its own franchise agreement with the cable company. All Cable TV companies today serve multiple towns and hence they are all MSOs.
The term likely became used back in early cable days when cable companies were founded to serve just one town. Soon these mom and pop cable companies started to consolidate and there we get MSO.
Russ C