Source : Free On-Line Dictionary of Computing

Generic Routing Encapsulation
     
         (GRE) A {protocol} which allows an
        arbitrary network protocol A to be transmitted over any other
        arbitrary network protocol B, by encapsulating the {packets}
        of A within GRE packets, which in turn are contained within
        packets of B.
     
        Defined in {RFC 1701} and {RFC 1702} (GRE over IP).
     
        (1998-07-19)