Mac OS 9 is the final version of what Apple would call Classic Mac OS and was released prior to the introduction of Mac OS X, now known as macOS. Apple would include what they called "Classic Environment" in Mac OS X from the new operating system's initial release up to 10.4 "Tiger", but that support ended at Mac OS X 10.5 "Leopard", and was never developed to work with Intel-based Macs.
The Classic Environment allowed Mac OS X users to continue to use Mac OS 9 applications inside of the new operating system so they might manage the transition without losing access to applications compatible only with Mac OS 8 & 9. Much of this compatibility was facilitated by Apple's Carbon system extension which allowed developers to quickly adapt their apps to run on both Classic Mac OS and Mac OS X. Carbon would be discontinued on the release of Mac OS X 10.15 "Catalina".
Steve Jobs, at Apple's 2002 World Wide Development Conference a few months after Mac OS 9 was officially discontinued, held a mock funeral for Mac OS 9, but fans of the OS just wouldn't let it die.