The Collaborative International Dictionary of English v.0.48:

Public-service corporation \Public-service corporation\ or
   sometimes Quasi-public corporation \Quasi-public corporation\
   A corporation, such as a railroad company, lighting company,
   water company, etc., organized or chartered to follow a
   public calling or to render services more or less essential
   to the general public convenience or safety.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

public-service corporation
    n 1: a company that performs a public service; subject to
         government regulation [syn: utility, public utility,
         public utility company, public-service corporation]