Source : Free On-Line Dictionary of Computing

backbone site
     
        A key {Usenet}, {electronic mail} and/or {Internet} site; one
        that processes a large amount of third-party traffic,
        especially if it is the home site of any of the regional
        coordinators for the {Usenet} maps.  Notable backbone sites as
        of early 1993 include {uunet} and the mail machines at
        {Rutgers University}, UC Berkeley, {DEC}'s Western Research
        Laboratories, {Ohio State University} and the {University of
        Texas}.
     
        Compare {rib site}, {leaf site}.
     
        [{Jargon File}]
     
        (1994-11-28)