Source : Free On-Line Dictionary of Computing

legacy system
     
         A computer system or {application program} which
        continues to be used because of the cost of replacing or
        redesigning it and often despite its poor competitiveness and
        compatibility with modern equivalents.  The implication is
        that the system is large, monolithic and difficult to modify.
     
        If legacy software only runs on antiquated {hardware} the cost
        of maintaining this may eventually outweigh the cost of
        replacing both the software and hardware unless some form of
        {emulation} or {backward compatibility} allows the software to
        run on new hardware.
     
        (1998-08-09)