Source : Free On-Line Dictionary of Computing

run-time library
     
         A file containing
        routines which are linked with a program at {run time} rather
        than at {compile-time}.  The advantage of such {dynamic
        linking} is that only one copy of the library needs to be
        stored, rather than a copy being included with each
        {executable} that refers to it.  This can greatly reduce the
        disk space occupied by programs.  Furthermore, it means that
        all programs immediately benefit from changes (e.g. {bug}
        fixes) to the single copy of the library without requiring
        recompilation.  Since the library code is normally classified
        as read-only to the {memory management} system, it is possible
        for a single copy of the library to be loaded into memory and
        shared by all active programs, thus reducing {RAM} and
        {virtual memory} requirements and program load time.
     
        (1997-07-16)