Source : Free On-Line Dictionary of Computing

Port Address Translation
     
         (PAT) A function provided by some {routers} which
        allows {hosts} on a {LAN} to communicate with the rest of a
        network (such as the {Internet}) without revealing their own
        private {IP address}.  All outbound {packets} have their IP
        address translated to the routers external IP address.
        Replies come back to the router which then translates them
        back into the private IP address of the original host for
        final delivery.
     
        Compare {SOCKS}.
     
        (1998-05-08)