The Free On-line Dictionary of Computing (30 December 2018):
Any software required to support the
production or execution of application programs but which is
not specific to any particular application.
System software typically includes an operating system to
control the execution of other programs; user environment
software such as a command-line interpreter, window
system, desktop; development tools for building other
programs such as assemblers, compilers, linkers,
libraries, interpreters, cross-reference generators,
version control, make; debugging, profiling and
monitoring tools; utility programs, e.g. for sorting,
printing, and editting.
Different people would classify some or all of the above as
part of the operating system while others might say the
operating system was just the kernel. Some might say system
software includes utility programs like sort.