Formerly, 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, when this sense of the term was beginning to pass out of general use due to wide availability of cheap Internet connections, included uunet and the mail machines at Rutgers University, UC Berkeley, DEC's Western Research Laboratories, Ohio State University, and the University of Texas. Compare leaf site.
[2001 update: This term has passed into history. The UUCP network world that gave it meaning is gone; everyone is on the Internet now and network traffic is distributed in very different patterns. Today one might see references to a “backbone router” instead —ESR]