backbone site

backbone site n.  A key Usenet and email 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}.