The Macintosh system software refers to system software used by the Macintosh from the very first version (System 1 in January 1984) all the way up to System 7.5, and frequently includes everything until Mac OS 8.
Before the advent of the Mac OS logo starting from Mac OS 7.5.1 (also known as System 7.5.1), there was no real name to give to the Mac's operating system. The term Macintosh system software was therefore used.
Core components of the Macintosh system software
(All of them form part of the System Folder)