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 1991 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.