backbone site: n. A key USENET and email site; one that processes a large amount of third-party traffic,

: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}.
-- The AI Hackers Dictionary