MSO stands for Multiple System Operator and is the industry term for cable company.
Each cable TV installation run by a system operator and serving a community is known as a cable system. Most system operators run cable systems in more than one community making them Multiple System Operators.
Increasingly MSOs are also being referred to as Multiple Service Operators, reflecting a shift in the industry. Cable companies started off as television service providers, but over the past years they have expanded their palette of services to include Internet, voice and data services. Many cable companies today are also full-fledged telecommunications service providers such that there is no longer a significant distinction between common carriers and MSOs.