What is Public Utility?

Legal Definition
A public utility (usually just utility) is an organization that maintains the infrastructure for a public service (often also providing a service using that infrastructure). Public utilities are subject to forms of public control and regulation ranging from local community-based groups to statewide government monopolies.

The term utilities can also refer to the set of services provided by these organizations consumed by the public: electricity, natural gas, water, and sewage. Broadband internet services (both fixed-line and mobile) are increasingly being included within the definition.
-- Wikipedia
Legal Definition
The provider of a service to the public such as transport, enertgy, telecommunications, waste disposal, or water and any other public goods and services.
Legal Definition
Any person, firm or corporation engaged in the business of rendering some service to the general public, e. g., common carrier, water, gas and telephone companies.
-- Ballentine's Law Dictionary