Wiktionary
n. (context computing English) The software associated with an operating system and supporting utility, database systems etc.
Wikipedia
System software (systems software) is computer software designed to provide services to other software. Examples of system software include operating systems, computational science software, game engines, industrial automation, and software as a service applications.
In contrast to system software, software that allows users to do things like create text documents, play games, listen to music, or web browsers to surf the web are called application software.
The line where the distinction should be drawn is not always clear. Many operating systems bundle application software. Such software is not considered system software when it can be uninstalled usually without affecting the functioning of other software. Exceptions could be e.g. web browsers such as Internet Explorer where Microsoft argued in court that it was system software that could not be uninstalled. Later examples are Chrome OS and Firefox OS where the browser functions as the only user interface and the only way to run programs (and other web browser can not be installed in their place), then they can well be argued to be (part of) the operating system and then system software.
Another borderline example is cloud based software. This software provides services to a software client (usually a web browser or a JavaScript application running in the web browser), not to the user directly, and is therefore systems software. It is also developed using system programming methodologies and systems programming languages. Yet from the perspective of functionality there is little difference between a word processing application and word processing web application.
Usage examples of "system software".
Power-on passwords are a function of the computer hardware, not the operating system software.
Erasing the program he'd had her input on her machine would be difficult for a CalTech-trained genius, but you could accomplish the same thing by clobbering the whole hard drive and reinstalling new files over the old ones, because the valuable little gopher file was hidden in the system software, and a write-over would destroy it as surely as the San Francisco Earthquake.
The system software had not yet learned to distinguish clutter from the purposeful activities of a human mind, at least not in all cases, and not, unfortunately, at this frequency setting….
THIS IS THE DEAL: I CONTRACTED WITH AMMATH TO OVERHAUL THEIR SYSTEM SOFTWARE, WHICH JOB I GOT BECAUSE I HAVE A DOD CLEARANCE FROM WHEN I WAS AT JPL.
Instead, the Satarran spy had actually rewritten some of the operating system software, reconfiguring it for his needs and yet still maintaining interface compatibility with the rest of the system.