Source : Free On-Line Dictionary of Computing

protocol analyser
     
         Any
        device that captures and interprets the {network traffic}
        between two or more connected computer systems.  The traffic
        can then be {decoded} so that it is possible to see what
        {processes} are occurring.
     
        By examining the flow of traffic, protocol analysers can be
        used to find out where problems (such as {bottlenecks} or the
        failure of a network device) are on a {LAN}.
     
        Advanced protocol analysers can also provide statistics on
        the traffic that can help to identify trends that may in
        future lead to further problems with the network.
     
        (2000-07-24)