Source : Free On-Line Dictionary of Computing

maximum segment size
     
         (MSS) The maximum amount of {TCP} data that a
        {node} can send in one {segment}.  This should be the size of
        the receiver's reassembly buffer to try to avoid
        {fragmentation}.
     
        The equivalent at the {physical layer} is "{Maximum
        Transmission Unit}".
     
        (1998-03-06)