Source : Free On-Line Dictionary of Computing

leaf site
     
        A machine that merely originates and reads {Usenet} news or
        mail, and does not relay any third-party traffic.  Often
        uttered in a critical tone; when the ratio of leaf sites to
        backbone, rib, and other relay sites gets too high, the
        network tends to develop bottlenecks.  Compare {backbone
        site}, {rib site}.
     
        [{Jargon File}]