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

run-time system
run-time support

    (RTS, run-time support, run-time) Library code
   and processes which support software written in a particular
   language running on a particular platform.  The RTS
   typically deals with details of the interface between the
   program and the operating system such as system calls,
   program start-up and termination, and memory management.

   (1999-07-26)