an organization supplying water, electricity, transportation, etc. to the public, operated, usually as a monopoly, by a private corporation under governmental regulation or by the government directly
See public utility in American Heritage Dictionary 4
noun
A private business organization, subject to governmental regulation, that provides an essential commodity or service, such as water, electricity, transportation, or communication, to the public.
Stock shares issued by a company providing essential public services. Often used in the plural.