The Free On-line Dictionary of Computing (30 December 2018):

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)